You could also check in at the Bahia Honda State Park
Looe Key Snorkeling Tours | Bahia Honda State Park ... they run short snorkeling tours ... about 1.5 hours it says, so that may not be all that challenging.
But there are multiple dive shops on pretty much every key throughout the island chain, so you can easily get answers ... but even on the sand beaches, they quickly give way to grass and coral patches that are not overly sandy ...
... if you are feeling really adventurous and in good shape, you could always dip into one of the channels ... but be sure there are no boats around, mind the currents and make sure the resident hammerheads are following the tarpon and not you ...
Also, here is the official tourism site for the Florida Keys -
Florida Keys Scuba Diving Vacation planning | Florida Keys Snorkeling - that site can connect you with a lot of the dive shops and dive operations down there ... they can at least answer questions for you.