About 70 miles west of Key West there's the Dry Tortugas. This was the best snorkeling we've done by far. From Key West we took a ferry, although you have the choice of a sea plane, as well, which is a much shorter trip. Our ferry ride took 2 1/2 hours but it didn't seem that long because the water was not choppy, we had smooth sailing and we saw awesome marine life on the way, including turtles!
In the Dry Tortugas there are a few designated swimming/ snorkeling areas. Two of these areas are around huge piles of old docks that had collapsed into the water. They made wonderful artificial reefs-- there was an abundance of many, many types of fish. We even saw an octopus! Large tarpon, barracuda, grouper were plentiful. Starfish, conch and a huge array of smaller fish were all there.
The water was very, very clear.
The Dry Tortugas has much history and a large part of the history there is the Fort that was built there in the 1800's. The Fort has a moat and all along the moat wall there's beautiful coral that has formed and, of course, the fish that love swimming among the coral. The Fort is very interesting to tour once you are done snorkeling. You should definitely do some reading up on the history of the Dry Tortugas to make the trip even more meaningful and worthwhile.
Lastly, you can swim a distance out from the shore, almost to the buoy and there are beautiful coral heads. Worth the swim. But we agreed that swimming among the dock pilings was the most exciting.
Another beautiful bonus was the birds that make these islands their home. We'd come up from snorkeling and see so many perched on top of the dock pilings.
One drawback is the price (ferries range from $145 to $165 per person. Breakfast, lunch, soda & water are included. Sea plane is around $250). But it's a wonderful, worthwhile trip. You won't be disappointed.