There are two Buck Islands in the Virgin Islands. One is a very famous park/nature preserve off the coast of St Croix. This is NOT where the tours go from St Thomas.
The other is a small island about 4 miles outside the harbor of St Thomas. That is the one the tours go to out of St Thomas. You cannot get on the island. Tours go to one of two coves. Turtle cove is a sandy cove with nice fish and an occaisional turtle (therefore the name). Shipwreck cove has the wreck of an old ferry in about 25 feet of water (again the name). Snorkelling is OK (but not great) at both . I have been to both. I like the tours as much for the boat ride, sun and fresh air as I do the snorkelling.