Mariah and the Billionaire -- We're Already Talking Marriage!!! (PHOTOS)

Mariah Carey doesn't waste time ... just days after she hopped on a yacht with new billionaire BF James Packer, they're already talking marriage.

Our Mariah sources say it's insane ... they've been "dating" for just a matter of days, but he's already told her he wants to marry her, sooner rather than later. The soonest can be when she's officially divorced from Nick Cannon. But we're told Mariah is open to tying the knot with James.

We got this photo of Mariah and James on the boat, wearing matching robes ... almost like they're on a honeymoon.

They've been cruising from country to country since late last week, and James blared Mariah's music for everyone to hear, including all the people on vastly smaller boats.

We broke the story ... James is obsessed with Mariah and her music. He went to her concert in Vegas a month-and-a-half ago and has been pursuing her ever since.