Out of the list of ports, the best bet for snorkling is the stop for St. Thomas. As johnny mentioned, Trunk Bay is an option. Either book through the ship, or if you have your own snorkle & mask, you can do it on your own by taking a cab to Red Hook, take the ferry, cab on the otherside to the USVI national park. It does take some time getting there but definately worth it. Don't want to snorkle all day? My 2nd recommendation would be Sapphire Beach. They have some shallow areas right off the beach where you can snorkle. Its a great spot for beginners and Novice's. You can rent equipment here. They have chairs for rent and water sports. 3rd - Coki Beach is fine for snorkling too.
St. Maarten - leave the snorkle gear on the ship. It's surrounded by sand. San Juan - stops there are short and I don't have experience snorkling there. I would say do a ship tour if they offer it. Antigua - mentioned that in another post. Nassau - yes. Take a tour through the ship. They have boat tours that can take you out to some of the Cays. There is some nice snorkling sites.
Johnny - The Radiance is a nice ship. It's one of my favorites
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