Heidi Klum living it up and stripping down on a beach in St. Barts ... wearing a tiny black bikini while splashing around with her husband, Tom Kaulitz.

The model enjoyed some sun, surf and sand Saturday ... going for a quick dip and engaging in a whole lot of PDA with her man -- sunglasses still covering her eyes even in the water.

After the two got their fill of the saltwater, they headed for the shore ... revealing Heidi's whole ensemble.

HK's decked out in a tiny top and thong-style bikini bottom ... leaving little of her body to the imagination.

We've told you all about Heidi and Tom's St. Bart's vacay ... with the two regularly hitting the water over the past few months -- and, Heidi even standing topless on the sand.

Heidi's talked about her love for going topless a lot ... admitting on the "Call Her Daddy" podcast last year that her kids always have to tell her to cover up when their friends come over.

Heidi and Tom have been vacationing in all kinds of climates recently ... with Heidi posting a sensual lingerie video from a freezing balcony near Aspen last month.

Bottom line ... Heidi's not afraid to strip down in any weather -- and, her fans are sure to be grateful for that!