The #1 ranked PGA player in the world decided to thong out for a friend's birthday party in Florida on Friday -- and it's clear ... dude ain't shy about it!!!

The 29-year-old butt-flossed up and bared his bod while drinking beer on a PonTiki boat in South Florida ... and tested the thong durability when he jumped off into the water.

The theme of the party seemed to be some sort of redneck vibe -- at one point, Brooks was rockin' a shirt that read, "Save a Horse, Ride a Cousin."

BK's hot girlfriend, Jena Sims, got in on the costume fun -- sporting a bathing suit inspired by that neon green man-thong from "Borat."