I've done Buck island twice. There are two anchorages. One has a sandy bottom and access along the shore line to some old coral. Most of the time you will see nice fish and once in a while a sea turtle. The other anchorage is deeper and there is less to see accept there is an old ferry or tug boat wreck. Not much color or fish. I'd rate the snorkeling here as Ok. (this is NOT the Famous Buck Island preserve which is off St Croix).
Wild Thing is a huge over powered speed boat. It takes you on a high speed tour of the island and snorkeling (usually at the same place as the Buck Island tour).
If you are truly interested in snorkeling take a tour (or go on your own) over to St. John.
Almost any cove or Bay is a snorkelers paradise. The most popular and populated will be Trunk Bay.