Justin Bieber -- Ass Ahoy, Matey!! Full Moon in Paradise (PHOTO)

Justin Bieber's fully naked ass hanging out on a boat deck ... or the gorgeous deep blue water of Bora Bora -- what are you really staring at in this pic?

It's okay. We'll give you a moment to think about your answer.

Justin just pulled his own version of "Break the Internet" ... with this nude pic of himself standing on the deck of a boat in the world famous lagoon.

Still thinking? We know. Stupid island pics are a dime a dozen.

Apply sunscreen liberally, Biebs!