It is (http://mappery.com/map-of/Map-of-Bimini-Islands-in-Bahamas) in fact the Fountain of Youth (http://www.conchcruisers.net/foy.htm). Or at least, it's a freshwater well with a plaque next to it saying it's the fountain of youth. Congrats to mangosteen who was the first to identify it as such.