While diving, and particularly in areas of swell, wave action and surge, our senses and equilibrium are assaulted continuously by sensations that should cause disorientation, unbalance, and vertigo while in the water. Add this to the motion of the boat and sea sickness is not an unusual result (even for those who are not normally susceptible to motion sickness).
I have experienced the same thing from time to time. I have taken to using Dramamine – Less Drowsy formula which really helps.
Another consideration is that you might be experiencing gastric upset due to the buildup of digestive gases during the dive and then expansion of those gases as you ascent. In order to counter this effect, I usually am very careful about what & when I eat prior to diving. I also eat a couple of Saltine or (soda cracker) just prior to diving. This appears to help stem the gas buildup.
For additional information you may want to consult your doctor or visit Divers Alert Network at http://diversalertnetwork.org/