Dives Way off Key West
Dry Tortugas National Park
The Dry Tortugas lie 60 miles off the tip of Key West, and have been protected by National Park status now for some years. The old fort-then-prison remains, and you can walk around the place, or better yet snorkel the flats around the old structure. The Gulf Stream keeps the waters around Dry Tortugas very clear, with visibility of up to 100 feet. It truly is a snorkeler's paradise. There are eight islands that make up this chain, sort of like a bunch of turtles wallowing in the water, their exposed shells drying in the sun. Indeed, the "Tortugas" is Spanish for "turtles".
The waters between Key West and Dry Tortugas as rough and the current is strong, so don't try to reach the spot on your own in a small boat. Either go with a charter or if you have your own large boat that is self-sufficient, go for it. There are no facilities at the park, so bring your own water, and have enough fuel for the trip back, of course!
Since the area is a National Park, it is against the rules to collect or capture any marine life, which means there is lots of it to see. There are 100 square miles of living coral reef to explore here, so you will want to come back again and again. Snorkelers will love the entire area, but Loggerhead Key in particular seems made for snorkeling. Also, there are numerous wrecks offshore of the Dry Tortugas, including a 300-foot steel-hulled wreck about one mile off shore.