Sombrero Reef, Florida Keys

Sombrero Reef is one of the best spots in the Florida Keys for snorkeling - you take a boat trip from Marathon (costs about $30 per person).
It does get quite busy, but the sheer amount of coral and life here makes it worthwhile: Gorgonians, Brain Corals, Lettuce Corals etc.
Turltles, Nurse Sharks, Rays, Barracudas, Groupers and Parrot Fish.
Water temperature ranges between about 22 degrees C in Winter and about 30 degrees C in Summer (72-87F).