I just returned from St. Thomas on the "Freedom of the Seas." I was at St. Thomas last year, both times I went on a snokeling excursion. I'm not impressed with snokeling at St. Thomas. Best I've had in snokeling has been the Caymans. I've been told Belize and the reefs around Cozumel are the best. St. Thomas was disappointing.
If you're just asking about snorkeling just in St. Thomas, I like Trunk bay, and Coki beach. It's not like snorkeling around a live reef, like in Belize. But at least it's a place to snorkel and see some fish.
Trunk Bay, St. John is okay sometimes but certainly not the best the times that I've been there anyway, though the beach and setting are surreal and definately worth going if you want the beach and snorkeling. The best snorkeling I've done while visiting St. Thomas was on an excursion to St. John's Honeymoon Beach. Went on this two times and it was absolutely the best I've done anywhere on either island. Most cruise ships offer the same excursion, it's called the Champagne Catarmaran Sail and Snorkel to St. John. It's ran by a captain from Boston and he and his crew do their best to keep the rum flowing (after the snorkeling ) The sites are fantastic sailing out and back and they stop so close to this heavenly deserted beach with blinding white sand palm fringed beach, shallow coral reefs everywhere. You can swim up to the beach and look out and see the fish teaming in the water, loads and loads of diferent types of fish. From the beach you can look out and see St. Thomas and other islands in the distance. Honeymoon Beach is located around the corner from the Caneel Bay Resort. The company who runs it is Atlantis Submarines but of course this is a cat not a sub. Holland America and Princess both charged $89/pp and Atlantis offers it for $69 according to their web site.
We took the Leylon Sneed to St. John--Trunk Bay--and consider this the single best day of vacation we've ever had. Husband, myself, teenage daughter, teenage son and his 2 friends. Beautiful scenery, great crew, great day and good snorkeling. Doesn't beat the snorkeling off Bahia Honda in the Florida Keys, but still good. Hope this helps.
Trunk Bay is beautiful, the snorkeling is good. Buck Island has good snorkeling. The best we've done so far is Dominica's Champagne Reef, Roatan had great snorkeling, and Bonaire. We haven't taken the seablaster excursion, so I don't know where you'd be snorkeling. You can't go wrong with either excursion. Best wishes and have a marvelous time!