Kylie Jenner Wet and Wild with Travis Scott for 22nd Birthday

Kylie Jenner is riding into 22 with a firm grip on her man -- she and Travis Scott did some tandem jet skiing as part of her outrageous birthday celebration at sea.

Kylie's moved the party into the Mediterranean Saturday -- her actual birthday -- off the coast of Italy. She and her baby daddy are cruising around with her closest GFs on their personal watercraft, as opposed to their OTHER "watercraft" ... the $250 million megayacht, "Tranquility."

And, check out the the birthday girl's sea-faring fashion sense -- an amazing technicolor swimsuit that definitely stands out ... as she always does.

The celebration's been going since the early hours of Saturday aboard their ship. KJ showed off one of her fabulous gifts from Trav ... a diamond chain with a massive pendant fashioned after her Kylie Lip Kit logo -- y'know that thing that made her so stinkin' rich.

It's a pretty sick piece, complete with liquid dripping off the lips ... all made of diamonds, of course.

Hey, we can't all be billionaires at 22, floating off the coast of Italy on a megayacht that costs $1.2 million per week to charter -- but at least we can all share in Kylie's fun.