From Divers Direct. Alligator Reef is one of the largest reef systems in the Upper Keys. In the 1960s, a study of Alligator Reef found and identified more than 500 species of marine life. The reef is around a wrecked ship; the USS Alligator built in 1820, which went down in 1825 while on a Navy mission. This 86-foot ship was attempting to protect a convoy from being raided by pirates. Salvaged after going aground, the crew decided it would be best to blow up the ship in order to protect her from further pirating. The remains of the ship are two piles of ballast stones but the surrounding reef is one of the largest in the area.
From American Dive Institute. Located 6 miles south-southwest of Windley Key and ranging in depths from 8 to 40 feet, this is an excellent dive for novice divers as well as those wanting photo opportunities of marine life. Mooring buoys make this dive very accessible. This is a SPA (Sanctuary Preserve Area) which covers .2 nautical miles. Alligator Reef is among the largest reef systems in the Upper Keys and is designated as a SPA area (Sanctuary Preserve Area). The 136 foot lighthouse tower off Windley Key is a good point of reference to find the site. Mooring buoys near the most popular part of the reef make anchoring easy. Nearby is Alligator Gulley which is a great site for spotting marine life and several species of coral. Corals and tropical fish are abundant.