Leonardo DiCaprio and Vittoria Ceretti Strip Down For St. Barts Beach Day

Leonardo DiCaprio and Vittoria Ceretti are letting the waters of St. Barts wash all the stress away ... diving into the crystal clear ocean in barely any clothing.

The two lovebirds hit the beach Friday ... Leo in a pair of black board shorts and Vittoria in a revealing two-piece bikini leaving little to the imagination.

Leo and Vittoria swam around together ... engaging in some light PDA while treading water off the coast of the island country.

Lots of big smiles and laughs between the two ... looking quite carefree as their holiday continues on.

The actor and the model have been together for a long time ... first hooking up over the summer last year -- and, they've been on a pretty constant vacation since it feels like.

They've been to Italy, France, London and New York City together ... partying hard into the early morning hours, meeting each other's families and a whole lot more couply stuff.

Unclear when their vacation will come to an end ... though Leo's got quite a few projects listed on his IMDb page -- so, he's got to get back to filming soon enough.