The actor jumped into the Mediterranean during his sojourn to Cannes ... splashing around in the cool, blue water before working on his tan on the ship's massive deck.

BTW ... Leo's rockin' a man-bun in these pics -- unclear if it's for a role or just a style choice, but either way, Leo's definitely pulling it off.

Meanwhile, Vittoria's right by LD's side ... decked out in a tiny bikini to combat tan lines -- and, even seems to strip it off completely at one point, holding the thin material over her chest with just her hands.

The two weren't just alone on a romantic vacay BTW ... they were joined on the yacht by a group of friends -- though, we're sure the ship's big enough for everyone to get a little alone time.

Leo and Vittoria are reaching a serious couple milestone ... with outlets first linking the two last August -- so, they're right around the big one-year mark.

We've seen them sight-seeing in exotic locales, and there was even speculation the two were engaged floating around in March -- though that ultimately proved not to be the case.

Good to see Leo's back in the ocean, too ... remember, just a few weeks ago DiCaprio suffered a jellyfish sting -- and, he needed medical attention on his A-list ass!