Britney Spears Sizzles in a Tiny Bikini While Dancing on a Boat, on Video

Britney Spears is at it again ... but this time at sea in a new Instagram dance video -- naturally, in a teeny-tiny bikini that doesn't leave much to the imagination.

Brit posted the new clip to wrap up her weekend ... which she spliced together with other dance vids from her house -- but this one stood out, as she was getting down on a cruising boat!

As you can see for yourself ... Britney is once again giving it all to the camera -- this time in a sun hat, and a yellow and green bikini.

The boat makes this latest one unique ... not to mention the random dude standing in the back, who appears to be a crew member of the ship -- just keepin' a watchful eye on his passenger. It's not clear where the video was recorded.

Prior to the dance collection vid, Brit was posting tender moments on IG with her son Jayden ... including one of them together captioned "Went to church today !!! Sang and praised !!!"

As you know ... Jayden’s the younger brother of 19-year-old Sean Preston. Britney was largely estranged from both boys during their teen years as they lived with dad Kevin Federline.