I've been to St. Thomas a few times, but never did Coral World or the Submarine, though the latter sounds fun. Coral World is next to Coki Beach, easily the worst beach on St. Thomas. It's small and crowded. Magen's Bay is the best. If you want to snorkel and have time, I'd ferry over to St. John and get to Trunk Bay, or better yet, get on a boat for a snorkel or dive trip. The cruise ship might offer such a trip. I'm not much of a shopper. Downtown St. Thomas is interesting. Lots of historic old buildings and a zillion shops. The most fun things to do in the Virgin Islands, I think, is nosing around St. John.