That depends on what kind of snorkeling you are comfortable with. Do you prefer doing it on the beach, or off a boat? We went to Frommer's Resort in Belize (about 1 hour drive from the port) that rented equipment and had snorkeling off the beach. We did this with our kids. There was plenty of fish to see, but all pretty much the same kind.
Grand Cayman has a sting ray excursion that you should definitely do if you haven't already. That is quite an experience that includes snorkeling.
RCI Radiance of the Seas Sep 2009
NCL Sun March 2008
Carnival Destiny June 2006
Carnival Ecstasy June 2002
Carnival Elation August 2000
Celebrity Century June 1998
Carnival Carnivale August 1993