The Florida Reef runs offshore of the Keys and is the reason the islands matter to divers and snorkellers. Coral heads, spur-and-groove formations, seagrass beds, and shipwrecks shelter parrotfish, barracuda, rays, turtles, and reef sharks, with the easiest visitor access from Key Largo, Islamorada, Marathon, and Big Pine Key.
Conditions vary sharply with wind and visibility. Beginners should book reputable snorkel boats from John Pennekamp, Islamorada, or Marathon rather than guessing from shore. Divers have better choices, from shallow patch reefs to deeper wrecks, but should check current marine sanctuary rules before touching anything or anchoring.