I totally agree with Trunk Bay. There are also some great catamaran tours to Buck Island that have good snorkeling. Antigua, too many jelly fish for me. St. Kitts, I agree with Bob1958, not really very good. Princess Cay, not bad and I enjoyed the island as well. You can go out the back of the fence and there are some local vendors there. I think they're ok with you walking around as well. At least we didn't get yelled at
If the weather isn't good, don't go to Trunk Bay, but if it is do make the trip, it's awesome. If it's kind of yucky, Sapphire Beach and Coki Beach have decent snorkeling on the island of St. Thomas itself. Hope this helps.