If you want to do it on your own, St. Thomas is going to have better options and easier to get around than Tortola - Secret Harbor and Sapphire Beach are pretty good. The VI Ecotours Hassel Island tour has some good snorkeling by kayak and departs out of Frenchtown near the cruise ship wharves.

For pure snorkeling variety and accessibility, St. Thomas (USVI) is often considered better for beginners and families, while Tortola (BVI) is the premier choice for "off-the-beaten-path" adventurers. St. Thomas offers high-value shore snorkeling at spots like Coki Point and Secret Harbour, where vibrant reefs and schools of tropical fish are just steps from the sand. It also features Buck Island, a short boat ride away, famous for its shipwrecks and sea turtle populations. On the other hand, Tortola serves as the gateway to the British Virgin Islands' more remote sites; while the snorkeling at Brewers Bay is excellent, the real "gold standard" comes from taking a day trip to the The Baths on Virgin Gorda or the Indians and the Caves near Norman Island. If you want a "polished" experience with easy gear rentals and beach bars, choose St. Thomas; if you prefer a rugged, sailing-oriented experience where you can hop between hidden coves and pristine reefs, Tortola is the superior base.