One on my favorite things to do on a cruise in the Caribbean is to go snorkeling. If it is from the shore or off a boat it does not matter to me.  I even bring my own gear (fins, snorkel, mask) on my cruises. Particular places I loved to snorkel in the recent past include off the beach in Cozumel, from a catamaran to a wreck in Grand Cayman, and from a dive boat in Gran Turk.