At Sea Level Club
Similar to the mile high club in all ways except that this club is joined while on a cruise ship.
Wow the cruise was amazing; my girlfriend and I finally joined the at sea level club. This time we truly did move like the motion of the ocean!
Sea Level Club
sex on the deck of a boat, presumably at "sea level"
Bored with the formal tour, my girlfriend and I went to the marine bathroom and joined the Sea Level Club.