The Kansas City Chiefs superstar and his high school sweetheart, Brittany Matthews -- sporting a hot pink bikini -- were having some fun in the sun at the Lake of the Ozarks on Wednesday ... and decided to turn things up a notch.

The couple look closer than ever on the boat trip ... including some very nice hand placement and some smooches.

"PG-13❕ #oops😘," Brittany captioned their impromptu photo sesh.

Of course, Matthews is easily Mahomes' most vocal supporter ... screaming her head off for her man's team every game en route to winning the Lombardi trophy.

Mahomes and Matthews have been even more inseparable during quarantine ... spending the offseason training together and other cute couples things.

Further proof that it's good to be the champs!!

PS -- amazing choice of shades, Patrick.