Ditto on Pennekamp, snorkel friendly,,,,kinda hard to offer any other places(dive shops) down there, since the last yrs. storm(some of them closed up), I think Hens and Chicks is another good place to snorkle. If you are going out with a dive shop's boat, check with them b4 you get there, and see where they go,,,
Nothing really right off shore to see ,snorkling or otherwise, unless you dive under the bridges, and i would not recommend it ,if you are not sure of tide times, or fisherman above/around you.(there are a wild bunch of stuff that comes/goes at tide change, but would not recommend being in the water at that time, from the current , or the big critters passing thru.)Most all diving in the Keys is done out on the reefs, nothing you can get to w/out a boat.
Find some dive shops down there, ask around and see where most of them are going to. some places will take snorkelers out with a dive group, and i think there might even be places that do snorkel only,,,(i seem to remember a catamaran trip , snorkle only that made a day of it, lunch and all

but the name evades me now)
Good luck, and hope you enjoy the Keys!