Mariah Carey -- Hangin' Out with Brett Ratner On His Birthday! (PHOTO)

Mariah Carey celebrated the birthday of her boyfriend, King Brett Ratner Saturday ... aboard the USS Rich People.

TMZ broke the story ...Mariah and Brett are in full-blown couple mode, sailing around the Bahamas on a mega-yacht owned by one of Brett's billionaire friends.

It's interesting ... these 2 have been friends for a number of years and worked together on Mariah's music videos as well. But it's clearly turned into a lot more.