St. Thomas and Barbados are two of my favorites. I do Trunk Bay when in St. Thomas or take a catamaran or sailboat. Barbados we use the Tiami catamaran, we've done them 6 times. The last time we went to a beach that was near the Folkstone Underwater Marine park, the cab driver took us there. It was really nice and we snorkeled from there to the marine park and back. The last time we went to St. Maarten we did the TikiHut snorkel. The operators were great! The snorkeling was pretty good for St. Maarten, but the best part was there was no time limit. They'd let you stay as long as you liked and would then drop you off in town or back to the ship for shopping. St. Lucia is nice at the Petons, St. Kitts don't waste your money snorkeling.
Hope you have a marvelous cruise!