Posted June 10th, 2006, 02:13 PM
I live in St Thomas and recently went on a half-day catamaran. To me, a half day really isn't long enough - the catamarans are slow, so you don't get too far from St Thomas.
We went with Loveland (I know they service Carnival, but probably not under that name), and what made theirs different is the captain will take the non-snorkelers out in a dinghy so you can get closer to fish. Also, on a half day trip, they don't have much time to look for fish, so if there aren't fish at the first stop, they may not have time to get to another.
We went on a day when there weren't ships in town, so the crew was probably less formal with us than they would be with the cruise ship passengers, but they were very friendly and kept the drinks coming (rum punch).
Some of the other catamarans leave from an area that's closer to the cruise ships - I think Dancing Dolphins is one of them. I've heard nothing but good things about all the operators, so you'll probably have a good time no matter which one you go with. I'd definitely go for the full day, though.