The K.C. Chiefs superstar QB hit a boat party with teammate Travis Kelce, their girlfriends and some other buds in Lake Tahoe on Thursday ... and yeah, you jealous yet or what?!?!

The fiesta on the water looked AWESOME ... the sun was shining, the guys were shirtless and the football players' girlfriends were letting loose with twerking and beer chugging!!

There was also swimming, laughing and good times had by all ... and it DEFINITELY feels like someone just signed a $503 MILLION contract!!

Of course, Mahomes, Kelce and their loved ones have been partying for a few days now ... after Pat inked his record-breaking Chiefs deal, he and Kelce hit a private jet Wednesday to Tahoe for some golf action.

The two are slated to compete in the famous celeb American Century Championship golf tournament this weekend ... but it's clear there's going to be a side of partying to go along with the birdies and bogeys.