Mariah Carey's New BF -- You Motorboatin' Son of a Bitch (PHOTOS)

Mariah Carey's billionaire boyfriend went full throttle on his yacht -- getting a face full of her cleavage, and proving the old adage ... it's really good to own a yacht.

James Packer was putting on a motorboating clinic while MC chilled on his vessel. As we've been reporting, they've been cruising around Italy all week, and there's already talk of marriage.

If this up close look at the family jewels doesn't close the deal ... nothing will.