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.

Nice to see her happy.