Snorkel the Key Largo and Looe Key areas of the Florida Keys National Marine Sanctuary. Don't bother with snorkeling in Key West (topside attractions are far more entertaining).
Unless there are divers or freedivers in your party, you're probably better off with a snorkel tour operator than a dive operator. Ask where they go, how many people they carry, how much water time is allotted, and what's included in the way of gear, beverages, snacks, etc.
You'll get a feel for whether or not a particular operator is for you based on their responses to your questions. Enjoy!