Kylie Jenner and Timothée Chalamet’s Mexican vacay is heating up fast ... the lovebirds hit the waters in Cabo, with Kylie ditching the layers and rocking just a bikini top!

Check the pics -- Kylie was serving full heat in a skimpy black swim top, kicking back on a private patio Monday, scrolling on her phone while lounging with Timothée ... who clearly earned this vacay after snagging that Oscar nod for "Marty Supreme."

The pair have been together since 2023 -- and judging by this trip, they’re fully in their groove, carving out some low-key solo time to recharge, reset, and keep the romance smooth.

But it wasn’t long before Kylie swapped the swimwear for a full outfit ... with the pair hopping onto a yacht, as A-listers do, to keep the romance cruising.

Water way to kick off the New Year!!