Water Movements – White Water/Surge/Inlets and Outlets/Tidal Currents/Surf

Because of the force of water movement, a diver can become a hostage to the sea.

White water

This water is white because of the foaming effect of air bubbles. This dramatically interferes with both visibility and buoyancy, as well as implying strong currents or turbulent surface conditions. A diver in white water is a diver in trouble. Under these conditions, the recommendation is usually to dive deeper.


The to-and-fro movement of water produces disorientation and panic in inexperienced divers, who often try to swim against it. Other divers use the surge by swimming with it, then hold onto rocks or corals when the surge moves in the opposite direction. This approach may be detrimental to the ecology, but good for survival.

Inlets and outlets

Occasionally, there is a continuous water flow, because of a pressure gradient through a restricted opening, which can siphon and hold (or even extrude) the diver. It is encountered in some caves, blue holes or rock areas near surf (an underwater ‘blow hole’), in human-made structures such as the water inlets in ships’ hulls and in outlets in dams and water cocks (taps). The pressure gradient may slowly draw the diver into its source and then seal him or her in, like a bath plug. Protection is by not occluding these inlets and by avoiding the area or covering it with a large grating.

Tidal currents

These currents are very important to the diver. If used correctly, they take the diver where he or she wants to go. Otherwise, they are likely to take the diver where he or she does not want to go. The latter event can be both embarrassing and terrifying, and it can also be very physically demanding.

Frequently, divers are lost at sea because of currents. Sometimes these currents can be vertical and cannot be combated by swimming or buoyancy. Certain popular diving areas, such as at Palau (especially Pelalu), Ras Muhammad, the Great Barrier Reef and Cozumel, are famous for their currents, and multiple fatalities are not uncommon.

Divers sometimes relate their successful swims against 4- to 5-knot currents. In fact, the average fast swim approximates 1.2 knots. For brief periods, it may be possible to reach up to 1.5 knots. The average swimmer can make very slow progress or none at all against a 1-knot current. A half-knot current is tolerable, but most divers experience this as a significant problem, and so it is. They tend to exaggerate the speed of the current as the hours go by, and especially during the après-dive euphoria (1 knot = approximately 2 km/hour).

Tidal currents are usually much faster on the surface than they are on the sea bed because of friction effects. A helpful observation is that the boat will usually face the current with its anchor upstream and the stern of the boat downstream. Any diver worth his or her salt knows that it is safer to swim against the current for the first half of the usable air and allow the current to bring the diver back to the boat for the second half of the dive. The ‘half-tank rule’ is worked out by taking the initial pressure, say 200 ATA, subtract the ‘reserve’ pressure (the pressure needed to charge the regulator), say 40 ATA, i.e. 160 ATA, and divide this by 2, i.e. 80 ATA. Thus, for this example, 80 ATA is used on the outward trip, and then the return is made with ample air to allow for misadventure (e.g. navigational error).

Untrained divers tend to make unplanned dives. They submerge and ‘just have a look around’. While they are having their look around they are being transported by the current, away from the boat, at a rate of 30 metres every minute in a 1-knot current. When they consider terminating the dive, after they have used most of their air, they have a very hard return swim against the current. They surface, because of their diminished air supply, well downstream from the boat and have to cope with a faster, surface current. This is a very difficult situation and far more hazardous, than that of the experienced diver who used the half-tank rule, who surfaced upstream from the boat and floated back to it, but who also had enough air to descend underwater and return with ease if desired or to rescue a companion.

The lines attached to the boat are of extreme importance when there are currents. First, there is the anchor line, and this is the recommended way to reach the sea bed upstream from the boat. The anchor chain should not be followed right down to the anchor because this may occasionally move if the boat moves, and it can cause damage to the adjacent divers. More than one diver has lost an eye from this ‘freak accident’. How may the diver reach the anchor line? A line may be attached to the top of the anchor line, with the other end to the stern of the boat. It should have enough play in it to allow divers to sit on the side of the boat and to hold it with one hand – the hand nearest the bow of the boat – while using the other hand to keep the face mask and demand valve in place. On entry, the diver ensures that he or she does not let go the line. The diver then pulls himself or herself forward to the anchor line and descends.

Perhaps the most important line, if there is a current, is a float line or ‘Jesus’ line. This line drags 100 metres or more behind the boat, in the direction of the current, and it has some floats to ensure that it is always visible to divers on the surface. It is often of value to have one diver on this line while the others are entering the water. The diver on the line virtually acts as a backstop to catch the odd stray diver who has not followed instructions and is now floating away with the current. The Jesus line is also of immense value at the end of the dive when divers have, incorrectly, exhausted their air supply or when they come to the surface for some other reason and find themselves behind the boat. This would not have happened had a dive plan been constructed and followed correctly. Occasionally, however, it does happen to the best divers, and it is of great solace to realize that the Jesus line is there and ready to save the sinner – irrespective of religious persuasion.

Even divers who surface only a short way behind the boat in a strong surface current may find that it is impossible to make headway without a Jesus line. If this is not available, they can descend and use their compass to navigate back to the anchor line or inflate the buoyancy compensator, attract the attention of the boat lookout and hope to be rescued.

Buddy breathing while swimming against a strong current is often impossible. Even the octopus (spare) regulator is problematic at depth or when two people are simultaneously demanding large volumes of air, typical of divers swimming against a current. An alternative air supply (a reserve or pony bottle) is of value, if it has an adequate capacity.

In dive planning, there should be at least one accessible fixed diving exit, easily identifiable, that serves as a safe haven. This may be an anchored boat, in areas with tidal currents. The safety boat is a second craft – not anchored – and this, like any boat that is driven among divers, needs a guard on its propeller. To attract the safety boat, various rescue options include the following:

  • A towed buoy.
  • An inflatable 2-metre-long bag, called the ‘safety sausage’, to attract attention.
  • Pressure tested distress flare (smoke/light).
  • Personal floatation devices.
  • Personal electronic, sonic or luminous location devices.

Divers can now carry a personal location beacon or emergency position-indicating radio beacon (EPIRB), especially of value if diving in fast currents. These devices need to be pressure protected and are of value only once on the surface.

There are other problems with currents, and these are especially related to general boat safety and ensuring that there is a stable anchorage.

When the current is too strong or the depth or sea bed is not suited to an anchored boat, a float or drift dive may be planned. This requires extreme care in boat handling. Divers remain together and carry a float to inform the safety boat of their position. It allows the surface craft to maintain its position behind the divers as they drift.

The concept of ‘hanging’ an anchor, with divers drifting in the water near it and the boat being at the mercy of the elements, has little to commend it. The raising of the diver’s flag under such conditions, although it may appease some local authorities, is often not recognized by the elements, reefs or other navigational hazards, including moored boats.

Some currents are continuous, e.g. the standing currents of the Gulf of Mexico, the Gulf Stream off Florida and the Torres Strait, but tidal currents are likely to give an hour or more of slack water with the change of tide. At these times diving is usually safer and more pleasant because the sediment settles and enhances visibility. To ascertain the correct time for slack water, reference has to be made to the tidal charts for that area. The speed of the current can be predicted by the tidal height.


Entry of a diver through the surf is loads of fun to an experienced surf diver. Otherwise, it can be a tumultuous moving experience and is a salutary reminder of the adage ‘he who hesitates is lost’. The major problem is that people tend to delay their entry at about the line of the breaking surf. The diver, with all his or her equipment, is a far more vulnerable target for the wave’s momentum than is any swimmer.

The warning given to surfers, referring to water colour, is that ‘White is right but green is mean and blue is too’. This ensures that the surfer enters the surf and avoids rips. For the diver, it is the opposite. The diver may use the apparently calmer water to ride the rip into the ocean.

When the surf is unavoidable, the recommendation is that the diver should be fully equipped before entry and not re-adjust face masks and fins until he or she is well through the surf line. The fins and face mask must be firmly attached beforehand because it is very easy to lose equipment in the surf. The diver walks backward into the surf while looking over his or her shoulder at the breakers and also toward a buddy. The face mask and snorkel have to be held on during the exposure to breaking waves. The regulator must be attached firmly to the jacket, with a clip, so that it is easily recoverable at all times.

When a wave does break, the standing diver presents the smallest possible surface area to it; i.e. he or she braces against the wave, sideways, with feet well separated, and he or she crouches and leans, shoulder forward, into the wave. As soon as possible, the diver submerges and swims (in preference to walking) through the wave area. If the diver has a float, then this is towed behind. It should never be placed between the diver and the wave.

Exit should be based on the same principle as entry, except then the surf is of value. The wave may be used to speed the exit by swimming immediately behind it or after it has broken. The float then goes in front of the diver and is carried by the wave.

Night Diving

Because of the impaired visibility, extra care is needed for night diving. Emergency procedures are not as easy to perform without vision. There is a greater fear at night. For inexperienced divers it is advisable to remain close to the surface, the bottom or some object (e.g. anchor, lines). Free swimming mid-water and without objects to focus on causes apprehension to many divers.

Preferably the site should be familiar, at least in daylight, without excessive currents or water movements and with easy beach access – diving between the boat and the shore. On entry the diver sometimes encounters surface debris that was not obvious from the surface.

Any navigational aid needs to be independently lighted. This includes the boat, the exit, buoys, buddies and so forth. A chemoluminescent glow stick (Cyalume light) should be attached firmly to the tank valve, and at least two reliable torches should be carried. The snorkel should have a fluorescent tip. A compass is usually required. A whistle and a day-night distress flare are sometimes of great value in summoning the boat operator, who has not the same capabilities of detecting divers at night.

Marine creatures are sometimes more difficult to see. Accidents involving submerged stingrays and needle spine sea urchins are more likely.

Signals include a circular torch motion (‘I am OK, how about you?’) or rapid up and down movements (‘something is wrong’). The light should never be shone at a diver’s face because it blinds him or her momentarily. Traditional signals can be given by shining the light onto the signaling hand. Waving a light in an arc, on the surface, is a sign requesting pickup.

Kelp Diving

Kelp beds are the equivalent of underwater forests. Kelp can be useful in many ways to the diver. It allows a good estimate of clarity of the water by assessing the length of plant seen from the surface. The kelp blades indicate the direction of the prevailing current. In kelp beds there is usually an abundance of marine life, and the kelp offers other benefits such as dampening wave action both in the area and the adjacent beach. Kelp can be used as an anchor chain for people to use when they are equalizing their ears, as well as to attach other objects such as floats, diver’s flags, surf mats, specimen bags and so forth.

Giant members of this large brown algae or seaweed may grow in clear water to depths of 30 metres. The growth is less in turbid or unclear water. Kelp usually grows on hard surfaces, e.g. a rocky bottom, a reef or, for more romantic divers, a Spanish galleon. It is of interest commercially because it is harvested to produce alginates, which are useful as thickening, suspending and emulsifying agents, as well as in stabilizing the froth on the diver’s glass of beer (après dive, of course).

Kelp has caused many diving accidents, often with the diver totally bound up into a ‘kelp ball’ that becomes a coffin. The danger of entanglement is related to panic actions and/or increased speed and activity of the diver while in the kelp bed. Twisting and turning produce entanglement.

Divers who are accustomed to kelp diving usually take precautions to ensure that there is no equipment that can snag the strands of kelp; i.e. they tend to wear knives on the inside of the leg, tape the buckles on the fin straps, have snug quick-release buckles and not use lines. Divers descend vertically feet first to where the stems are thicker and there is less foliage to cause entanglement. The epitome of bad practice in kelp diving is to perform a head first roll or back roll because it tends to result in a ‘kelp sandwich with a diver filling’.

The kelp is pushed away by divers as they slowly descend and ascend; i.e. they produce a clear area within the kelp, into which they then move. They ensure that they do not run out of air because this situation will produce more rapid activity. If they do become snagged, divers should avoid unnecessary hand and fin movements. Kelp can be separated either by the use of a knife or by bending it to 180 degrees, when it will often snap (this is more difficult to achieve while wearing gloves). It is unwise to cut kelp from the regulator with a knife without first clearly differentiating it from the regulator hose. Some divers have suggested biting the strands with one’s teeth. This may be excellent as regards dietary supplementation, kelp being high in both B vitamins and iodine, but it does seem overly dramatic.

Kelp does float, and it can often be traversed on the surface by a very slow form of dog paddling or ‘kelp crawl’, in which one actually crawls along the surface of the water, over the kelp. This can be done only if the body and legs are kept flat on the surface, thus using the buoyancy of both the body and the kelp, and by using the palms of the hands to push the kelp below and behind as one proceeds forward. Any kicking that is performed must be very shallow and slow.

Fresh Water Diving

The main problem with fresh water is that it is not the medium in which most divers were trained. Thus, their buoyancy appreciation is distorted. Acceptable weights in sea water may be excessive in fresh water. Depth gauges are calibrated for sea water, and so they need to be corrected for diving in dams, lakes, quarries and so forth. Because these waters are often stationary, there may be dramatic thermoclines, requiring adjustments for thermal protection and buoyancy, as one descends.

There are also many organisms that are destroyed by sea water but that thrive in warm fresh water. Some of these, such as Naegleria, are fatal.

Deep Diving

‘Divers do it deeper’ represents a problem with ego trippers and a challenge to adventure seekers. Unfortunately, the competitive element sometimes overrides logic, and divers become enraptured, literally, with the desire to dive deeper. They then move into a dark, eerie world where colours do not penetrate, where small difficulties expand, where safety is farther away and where the leisure of recreational diving is replaced with an intense time urgency.

Beyond the 30-metre limit the effect of narcosis becomes obvious, at least to observers. The gas supply is more rapidly exhausted and the regulator is less efficient. Buoyancy, resulting from wetsuit compression, has become negative, with an inevitable reliance on problematic equipment, such as the buoyancy compensator. The reserve air supply does not last as long, and the buoyancy compensator inflation takes longer and uses more air. Emergency procedures, especially free and buoyant ascents, are more difficult. The decompression tables are less reliable, and ascent rates become more critical.

Overcoming some problems leads to unintended consequences. Heliox (helium-oxygen mixtures) reduces the narcosis of nitrogen, but at the expense of thermal stress, communication and altered decompression obligations. Inadequate gas supplies can be compensated by larger and heavier cylinders, or even by rebreathing equipment, but with many adverse sequelae (see Chapter 62).

Many of the older, independent instructors would qualify recreational divers only to 30 metres. Now, with instructor organizations seeking other ways of separating divers from their dollars, specialty courses may be devised to entice divers to ‘go deep’ before they have adequately mastered the shallows.

Cold/Ice Diving

The obvious problems are those of cold and hypothermia. They are so obvious that most people will avoid them by the use of heating systems, drysuits or efficient wetsuits. See Chapters 27 and 28 for the effects of a cold environment on physiological performance.

A major difficulty with cold and ice diving is the tendency of many single hose regulators to freeze, usually in the free-flow position, after about 20 to 30 minutes of exposure to very cold water (less than 5°C). This situation is aggravated if there is water vapour (potential ice crystals) in the compressed air and if there is a rapid expansion of air, which produces further cooling in both first and second stages. The first stage or the second stage may then freeze internally.

Expansion of air as it passes from the high tank pressure to the lower pressure demand valve and then to environmental pressures (adiabatic expansion) results in a drop in temperature. It is therefore not advisable to purge regulators if exposed to very cold temperatures. The freezing from increased air flow follows exertion, hyperventilation or panic. Octopus rigs become more problematic to use under these conditions, or at great depth, because of this increased air flow. An emergency air source (pony bottle) has replaced buddy breathing and octopus rigs.

‘External’ ice is formed in and around the first (depth compensated) stage of the regulator, thus blocking the orifice and interfering with the spring. Moisture from the diver’s breath or water in the exhalation chamber of the second stage may also freeze the demand mechanism, causing free flow of gas or ‘internal’ freezing with no flow.

Modifications designed to reduce freezing of the water in the first stage include the use of very dry air and the replacement of first-stage water-containing areas with silicone, oils or alcohols (which require lower temperatures to freeze) or with an air flow from the regulator. The newer, non-metallic second stages are less susceptible to freezing. Despite all this, regulator freezing is common in polar and ice diving. Surface supply with an emergency scuba, or twin tank–twin regulator diving, as with cave diving, is probably safer. It must be presumed in under-ice diving that the regulator will freeze and induce an out-of-air situation, and this must be planned for.

Under ice there is little use for snorkels, and so these should be removed to reduce the likelihood of snagging. Rubber suits can become sharp and brittle. Zippers are best avoided because they freeze and may also allow water and heat exchange. Buoyancy compensators should be small and with an independent air supply.

As a general rule, and if well-fitting drysuits are unavailable, the minimum thickness of the Neoprene should increase with decreased water temperatures, as in the following examples:

<5°C – 9-mm-thick wetsuit
<10°C – 7-mm-thick wetsuit
<20°C – 5-mm-thick wetsuit
<30°C – 3-mm-thick wetsuit

Hood, gloves and booties should be of a considerable thickness, or heat pads can be used. Heat pads must not be in contact with high-oxygen gases because overheating can result.

Unheated wetsuits do not give sufficient insulation at depth (beyond 18 metres) when the Neoprene becomes too compressed and loses much of its insulating ability. In that case, non-compressible wetsuits, inflatable drysuits or heated suits are required. In Antarctic diving, to gain greater duration, we had to employ a wetsuit or other thick clothing under a drysuit.

Ice diving is in many ways similar to cave diving. It is essential that direct contact must always be maintained with the entry-exit area. This should be by a heavy-duty line attached to the diver via a bowline knot. The line must also be securely fastened at the surface, as well as on the diver. The dive should be terminated as soon as there is a reduced gas supply or any suggestion of cold exposure with shivering, diminished manual dexterity and so forth.

The entry hole through the ice should be at least two divers wide. Allowing room for only one diver to enter ignores two facts. First, the hole tends to close over by freezing. Second, in an emergency two divers may need to exit simultaneously. There should be a surface tender with at least one standby diver. A bright light, hanging below the surface at the entry-exit hole, is also of value in identifying the opening. If large diving mammals contest the opening in the ice, they should be given right of way.

If the penetration under the ice is in excess of a distance equated with a breath-hold swim, then a back-up scuba system is a requirement, as with cave diving.

Cave and Wreck Diving

These enclosed environments are hazardous to open water divers. Cave diving and wreck diving are more complex than they first appear. Completion of the open water scuba training course is inadequate preparation for cave and wreck diving. Planning involves not only the setting of goal-oriented objectives, but the delineation of maximum limits (depths, distances). The main problems are as follows:

  • No direct ascent to the surface (i.e. safety).
  • Disorientation and entrapment.
  • Loss of visibility.
  • Enclosed spaces and panic.

Cave diving

The techniques of cave diving are very rigidly delineated. Specialized training includes dive planning, the use of reels and lines and the lost diver protocols. Most people who have difficulties with cave diving have not followed the recommended rules, and unfortunately cave diving problems tend to cause multiple fatalities.

The diver descends, often through a small access, passes down a shaft, goes around a few bends and is faced with multiple passages, in total darkness. Under these conditions, and to make this particular type of diving safe, it is necessary to be accompanied by a diver who has considerable cave experience – in that cave – and whose judgement is trustworthy. It is equally important that the equipment is both suited to cave diving and totally replaceable with spares during the dive. Apart from the obvious environmental difficulties inherent in diving through a labyrinth of passageways, there are added specific problems.

Safety in cave diving is not usually achievable by immediate surfacing. Thus, all necessary equipment must be duplicated for a long return swim, at depth, and possibly while rescuing a disabled companion.

Air pockets found in the top of caves are sometimes non-respirable because of low oxygen and high carbon dioxide levels (especially in limestone caves), so when entering this pocket, breathing should be continued from the scuba equipment. Sometimes the roof of the cave is supported by the water, and when this water is replaced by air from the diver’s tanks, the roof can collapse. The common claim that ‘the diver was so unlucky for the roof to collapse while he was there’ is incorrect. It collapsed because he was there.

The minimum extra safety equipment includes a compass, powerful lights and a safety reel and line. It is a diving axiom that entry into a cave is based on the presumption that the return will have to be carried out in zero visibility.

For visibility, each diver takes at least two lights; however, other factors can interfere with the function of these lights. A great danger is the silt that can be stirred up if the diver swims along the lower part of the cave or in a head-up position (as when negatively buoyant). If there is little natural water movement, clay silts can be very fine and easily stirred up. It is for this reason that fins should be small, and the diver should be neutrally buoyant and should swim more than a metre above the bottom of the cave. Visibility can be totally lost in a few seconds as the silt curtain ascends, and it may remain that way for weeks. Sometimes it is inevitable, as exhaled bubbles dislodge silt from the ceiling. Layering of salt and fresh waters also causes visual distortion and blurring.

The usual equipment includes double tanks manifolded together, making a common air supply, but offering two regulator outlets. With the failure of one regulator, the second one may be used for the air supply – or as an octopus rig. The second regulator must have a long hose, given that often divers cannot swim alongside each other. Because of space limitations, buddy breathing is often impractical under cave conditions. An extra air supply (‘pony’ bottle) is advisable.

For recreational divers to explore caves, the ideal equipment is a reliable compressed air surface supply, with a complete scuba back-up rig.

All the instruments should be standardized; e.g. the watch goes on the left wrist, the depth gauge above it, the compass on the right wrist and the dive computer (this can include a contents gauge, decompression meter, dive profile display, compass) attached to the harness under the left arm. The gauges and decompression must be modified for fresh water and altitude, if these are applicable. The knife is strapped to the inside of the left leg, to prevent entanglement on any safety lines.

The buoyancy compensator is often bound down at the top, to move the buoyancy centre more toward the centre of gravity (cave divers do not need to be vertical with the head out of water). There is no requirement for excess buoyancy because safety in cave diving is not usually equated with a direct ascent; thus, any carbon dioxide cylinders should be removed and replaced with exhausted ones to prevent accidental inflation of vests. A principle of cave diving is that safety lies in retracing the entry path by the use of lines and not by ascent, as in the normal open ocean diving.

Preferably no more than three divers should undertake a single dive, and on completion of the dive each should have a minimum of one third of the initial air supply. If there is water flow within the cave, and the penetration is with the flow, this reserve air supply may not be adequate because the air consumption is greater when returning against the current.

Vertical penetrations need a heavy shot line moored or buoyed at the surface and weighted or fixed at the bottom. The reel is used for horizontal penetrations, not vertical. Otherwise, entanglement is likely with rapid ascents, especially if divers precede the lead diver. Thin, non-floating lines especially cause entanglement if they are allowed to slacken.

Specialized cave diving training is a prerequisite for this diving environment.

Wreck diving

Wreck diving has potentially similar problems to some cave and ice diving. In addition, it has the hazards of instability of the structure and the dangers of unexploded ordnance, sharp objects, toxic cargo and fuel. Exhausted gas from scuba may cause air pockets and disrupt the wreck’s stability.

Silt in wrecks is usually heavier than that in still water caves. Thus, the sudden loss of visibility that can occur when silt is stirred up may be less persistent. The diver should ascend as far as is safe and wait until the silt cloud settles down.

Altitude Diving

The term altitude diving refers to diving at an altitude of 300 metres or more above sea level. Non-diving disorders should be considered, such as the dyspnoea and hypoxia induced by high altitude and the altitude sickness that frequently develops above 3000 metres. Diving at altitudes higher than this is strongly discouraged.

The following numerical examples do not represent actual diving conditions and are used to explain the problems as simply as possible, thus avoiding complicated mathematics. The conventional idea of diving is that a diver descends with the sea surface (1 ATA) as the reference point and returns there when he or she has finished the dive. A diver may have to dive at altitude, in a mountain lake or dam, where the pressure on the surface is less than 1 ATA. Problems stem from the physics at this altitude.

For simplicity’s sake, the following description is based on the useful, but not strictly correct, traditional theory that the ratio between the pressure reached during the dive and the final pressure determines the decompression required. If this ratio is less than 2:1, then a diver can ascend safely without pausing during ascent. This means that a diver from the sea surface (1 ATA) can dive to 10 metres (2 ATA) and ascend safely, as regards decompression requirements. A diver operating in a high mountain lake, with a surface pressure of 0.5 ATA, could dive only to 5 metres (1 ATA) before he or she had to worry about decompression. This statement ignores the minor correction required with fresh water. Fresh water is less dense than salt water.

Another pressure problem occurs when a diver, who dives at sea level, then flies or ascends into the mountains after the dive. For example, a 5-metre dive (1.5 ATA) from sea level could be followed by an immediate ascent to a pressure (altitude) of 0.75 ATA, with little theoretical risk. Deeper dives or greater ascents may require the diver to pause at sea level if the diver is to avoid decompression sickness. If the diver ascends, in a motor vehicle or an airplane, the reduced pressure will expand ‘silent’ bubbles or increase the gas gradient to produce larger bubbles, thereby aggravating the diseases of pulmonary barotrauma and decompression sickness.

Thus, exposure to altitude after diving, or diving at altitude, increases the danger of decompression sickness, compared with identical dives and exposures at sea level. It influences the decompression obligations, the depths and durations of decompression stops, the nitrogen load in tissues afterward, the safe durations before flying or repetitive diving, the ascent rates recommended during diving and so forth. Formulae are available to convert the equivalent altitude decompressions to sea level decompressions.

Another problem of diving in a high-altitude lake is the rate at which a diver may have to exhale during ascent. A diver who ascends from 10 metres (2 ATA) to the ocean surface (1 ATA) would find that the volume of gas in the lungs has doubled. Most divers realize this and exhale at a controlled rate during ascent. They may not realize that an equivalent doubling in gas volume occurs in only 5 metres of ascent to the surface, if the dive was carried out at an altitude (pressure) of 0.5 ATA. Equivalent effects are encountered with buoyancy, which can more rapidly get out of control at altitude.

The diver’s equipment can also be affected or damaged by high-altitude exposure. Some pressure gauges start to register only when the pressure is greater than 1 ATA. These gauges (oil-filled, analogue and mechanical types) may try to indicate a negative depth, perhaps bending the needle, until the diver reaches 1 ATA pressure. Thus, the dive depth would have to reach more than 5 metres before it even started measuring, if the dive had commenced at an altitude of 0.5 ATA.

The other common depth gauge, a capillary tube, indicates the depth by an air-water boundary. It automatically adjusts to the extent that it always reads zero depth on the surface. The volume of gas trapped in the capillary decreases with depth (Boyle’s Law). For a diver starting from 0.5 ATA altitude, this gauge would read zero, but it would show that the diver had reached 10 metres when he or she was only at 5 metres depth. Theoretically, the diver could plan the dive and decompression according to this ‘gauge’ depth, but only if he or she was very courageous.

Many electronic dive computers do permit correction for altitude, and some need to be ‘re-zoned’ at the dive site. Other decompression meters are damaged by exposure to altitude (e.g. as in aircraft travel), and the applicability of other dive computers to altitude diving or saturation excursions is questionable.

Divers who fly from sea level to dive at altitude, as in high mountain lakes, may commence the dive with an already existing nitrogen load in excess of that of the local divers, who have equilibrated at the lower pressures. Thus, the ‘sea level’ divers are in effect doing a repetitive dive, and ‘residual nitrogen’ tables must be employed.

Decompression tables that supply acceptable modifications for altitude exposure include the Buhlmann and Canadian Defence and Civil Institute of Environmental Medicine (DCIEM) tables (see Appendix A).

Altitude exposure and altitude diving are more hazardous extensions of conventional diving. They are not as well researched, and the greater the altitude, the more applicable is this statement. It includes not only the problems already mentioned, but also the complication of diving in fresh, often very cold, water. This water may contain debris that has not decomposed as it would in the ocean and may therefore threaten entrapment. The sites are often distant from diving medical facilities. Undertaking a specialized course in altitude diving is a basic prerequisite.

Undersea Environments

For the diver who is adequately trained and physically fit, who is aware of the limitations of the equipment and who appreciates the specific requirements of different environmental diving conditions, the sea is rarely dangerous. Nevertheless, it can be hazardous and unforgiving if attention is not paid to all these factors.
Diver training is specific to the environment in which the diver is trained. Specialized techniques are recommended to cope with different environments. They cannot be automatically extrapolated to other diving environments. The induction of fear in the inexperienced diver and of physical stress in the more skilled diver is appreciated only when one examines each specific environmental threat. These environmental stresses are mentioned in this and other chapters. The reason for including them in a medical text is that unless the physician comprehends the problems and dangers, the medical examinations for diving fitness and the assessments of diving accidents will be less than adequate.

Some aspects of the environments have physiological and pathological sequelae and therefore have specific chapters devoted to them. They include the effects of cold (see Chapters 27 and 28), altitude and fresh water diving (see Chapter 2), explosives (see Chapter 34), depth (see Chapters 2, 15, 20, 46 and 68) and marine animal injuries (see Chapters 31 and 32). Other environmental topics that are covered more comprehensively in diving texts are summarized in this chapter.

Being kept underwater and exceeding the limited air supply will result in drowning. This is a situation common to many of the hazardous environments, including caves and wrecks and under ice, overhangs, water flows and so forth. A variety of materials can trap the diver, including kelp, lines (even ‘safety’ lines), fishing nets and fishing lines. If the diver does not have a compromised air supply, then knowledge of the environment, a buddy, a communication facility, a calm state of mind and a diving knife or scissors will cope with most of these circumstances.