If you can spend all day, I'd suggest taking a ferry trip out to Dry Tortugas National Park/Fort Jefferson. The big attraction is a huge 19th century fort built on a tiny island. It appears to be rising out of the middle of the ocean, 90 miles west of Key West. There are two ferry services out there, and I've used both with success. You can snorkel at Fort Jefferson itself (only so-so). One of the operators has a side trip to a nice snorkeling site that includes an old sailing ship wreck. There are flyers for both services all over Key West. I just looked, and one of them has a web site at www.drytortugasferry.com. They also advertise other pure snorkeling expeditions, but I don't have personal experience with those.

There are seaplane trips out there, which are faster and should be breathtaking, but I don't have personal experience of them. Try www.seaplanesofkeywest.com.

Have fun!
Mike, the landlocked snorkel buff.....