Part of the fun of Fort Tilden Beach is exploring the abandoned bunkers. The concrete structures can serve as quick shelter from a sudden summer storm or a place to quickly change clothes (although some reek of urine) or, as this shot from DoubleBen shows, a place to lay your "sharkdick" tag.
Fort Tilden was sort of a "secret" city beach until recently, but it's been around forever, as this shot from 1988 shows. The njhunkguy photostream is full of similar pictures of him, in Speedos, on beaches around the world.