West of Key West by boat, you'll come across the popular snorkeling zones of Rock Key and Sand Key. The Rock Key area is east of Sand Key and it is completely submerged. Sand Key is a very tiny strip of sand poking out of the water, upon which sits an old lighthouse. Reefs are everywhere in this area, so it's no wonder they put up a lighthouse to ward ships off.
The lighthouse has an interesting history that you can read about using the link below. Near the lighthouse is a small marine observation buoy which happens to publish its data on the Internet.
I found the reef at Rock Key to be far superior to the one at Sand Key. While it was interesting to snorkel off of the lighthouse, the water was very deep and the coral was in poor condition. The Rock Key reef was much shallower and alive. My first snorkel trip ever was at Rock Key in December 1995, and I still think it's one of the best places I've been. Sand Key was probably the worst. It's amazing how different it can be over such a small distance.
Read more about the Sand Key Lighthouse.