We had a great time on the America's Cup Regatta in St. Maarten, that may be something that you would enjoy? Its one of those that does fill up quickly, so check in your shore excursion booklet and see what you think. Nice beach in St. Thomas is Meagans Bay, nice calm waters, but unfortunately not any good snorkeling there. If you really want to snorkel a trip over to St. Johns is the place to go either on your own or through a ship tour, a must see island. We also had fun on the "wild thing" a few years ago, another good snorkeling excursion.
I'm sure you've looked through all the excursions, lots to pick from. If you do see something you just have to do, try booking it on-line before you go, great feature. Tickets will be in your cabin when you arrive and then you don't have to worry about signing up once onboard, you'd be all set!