It was booze, butts and boats for Kristin Cavallari and Jay Cutler on their Mexican vacation ... with the two stars partying their asses off on a yacht with a gang of friends.

The Cutlers were in Mexico to celebrate Kristin's 30th birthday ... but the highlight was a booze cruise on a boat called the "Blue Guru" with celeb hairstylist Scotty Cunha.

Guessing Cutler paid for the excursion since he reportedly made $16 mil this year.

Not bad for a guy who didn't make the playoffs.