On our way back from Key West we stopped at Fiesta Key RV Resort for the night. There is a restraunt on the water -- you can access the water right near the restraunt. About 20 yds off the shore there is reef (kind of boring on the land side... but the other side full of life and a lot off fun). It gets shallow going over the reef 1-2' but just keep swimming and you'll get back into 5-6' off water.
We saw lots of wildlife -- a few lobsters and tons of fish. Right from the beach say 100-200 yards off at the most (beware: we also saw a small crocodile the marina a few hours later
I love diving the keys - great dive spots. But also most places down there are great for snorkelling. On a couple of my dives in key west, my wife and others could easily have come along and snorkelled. The reefs are beautiful and many are in 10-20' of water -- and the dive shops accommodate snorkellers and divers.