Well we went on one of the St. John catamaran - snorkling excursions. Unfortunately the weather was cloudy and a bit cool and the sea was pretty rough. My family did not have any problems with sea sickness, but there were problably 10 or 12 people loosing their breakfast over the side of the boat during the ride there. When we got to the cove where we were to snorkle, there were an awful lot of other boats there and the water was very cloudy from the rough seas and all the people snorkling. I felt like I was snorkling late in the day at CC. I'm sure that our experience may not be the norm for that excursion, but I don't think I would vote for it.
It's funny, but we had a much better expierence with a catamaran - snorkling excursion at Nassua on a 4 day cruise we took the year before.