I m no expert, but my wife and I took a day trip from Key West out to Dry Tortugas National Park and we had a great time touring the fort and snorkeling. It s the only time I ve ever been snorkeling though, so I don t really have anything to compare it to.
actually my opinion of florida keys snorkeling is that it isnt that great--i ve gone on a tour from key west and thought it kind of sucked--the fish were very few, the corla was not very vibrant, and the water wasnt even that warm--i m comparing this to some amazing spots i ve been to in hawaii and the bahamas...btw, dry tortugas is a good suggestion but it requires a $150 ferry ride to get there and it is just for a few hours around the national park