Almost everybody is susceptible to motion sickness1. In general, the population can be divided roughly into one third who are highly susceptible, one third who react only under rough conditions and one third who become sick only under extreme conditions. Although anyone with a normally functioning vestibular system is susceptible, people who are totally deaf and have unresponsive vestibular systems are very resistant.
In diving, two situations predispose to seasickness. The first is on the boat going to the dive site, and the second is while the diver is in the water, particularly if attached to the boat, for example, on a shot line during decompression. Most divers are less susceptible to seasickness while swimming underwater than when they are on the boat. For this reason, many divers hurry to enter the water after exposure to adverse sea conditions en route to the dive site. Problems develop because divers are inadequately prepared and equipped as a result of haste or from the debilitating and demoralizing effects of seasickness.