Florida Keys National Marine Sanctuary protects 2900 square nautical miles of waters surrounding the Florida Keys, from south of Miami westward to encompass the Dry Tortugas, excluding Dry Tortugas National Park. The shoreward boundary of the sanctuary is the mean high-water mark, essentially meaning that once you set foot in Keys waters, you have entered the sanctuary.
Within the boundaries of the sanctuary lie spectacular, unique, and nationally significant marine resources, from the world's third largest barrier reef, extensive seagrass beds, mangrove-fringed islands, and more than 6000 species of marine life. The sanctuary also protects pieces of our nation's history such as shipwrecks and other archeological treasures. - NOAA