Also one of my little pull off areas in the keys. There are also a couple of off shore reefs like coffins patch about 2 mile off marathon key.Sorry to reply so slowly, but we just got back from hiking in and around the Grand Canyon and the internet was VERY difficult to find!
I'm sure there are places to snorkel around Key West, but I've never done it. IMHO there are just too many people. The Horseshoe is a great place to snorkel and collect.
The inside is mostly deep water, but if you can free dive 15" or more, there are flame scallops and coral banded shrimp galore!
Out toward the open end of the quarry and along the inside edge to the left (SW) are the best areas. Picking up rocks to look under will get you to a lot of wildlife.
Also, there is now a walkway from the parking lot, around the NE end of the island (under the bridge) and out to a good snorkel site. But this one is far less protected against the prevailing SE wind, so it's only good on pretty calm days.
Have fun and now that I'm home, feel free to ask questions.