A nice place to snorkel in while you are in St. Thomas, would be to take a trip over to St. JOhn/Trunk Bay. This island itself is picture postcard beautiful and worth the time and effort to get over there. You can find a ship excursion or simply go on your own.
If you go on your own, just get a cab at the pier over to Red Hook ferry and then once at St. JOhn, another shorter cab ride to the beach. They do rent snorkel equipment, but if you have your own, just bring it along.
Another beach with a lot of water activities is Orient Beach in St. Maarten, although it is clothing optional right next door to this beach, so beware :-)