Greg, my husband I just returned from AC and did his dive certification and some snorkeling in Hol Chan. I am extremely susceptible to motion sickness. Hol Chan (and Shark Ray Alley) are inside the reef which makes them much, much calmer than sites outside the reef. When we were there, the island was experiencing very high winds (diving was actually cancelled for two days). Needless to say, the sea was choppier than normal. But it is always better inside the reef. The first day we went out, I had no problem with the seas. The second day, I got a little queezy after the first dive. It all depends upon how sensitive you (or your travel partner) are to motion sickness. If in doubt, take Bonine (much better than Dramamine). Take one the night before and 1 or 2 more (depending upon your body size) the next morning one to two hours before you go out. Avoid greasy foods and ginger also helps a bit. My recommendation, if the wind is at all strong, take the Bonine... better safe than seasick.