Michael Phelps Scores at the Pool ... with Miss Cali

Olympic god Michael Phelps showed off his skills in the pool ... mackin' on his on-again/off-again GF Miss California USA Nicole Johnson at the Grand Wailea in Maui yesterday.

We're told Phelps took a bunch of photos and signed autographs for fans ... in between gettin' real close and personal with Miss Cali.

The dude has got game.