Maisie Williams let loose during a vacation in Italy ... stripping down to nothing to take a dip in the Mediterranean Sea!

Check out the "Game of Thrones" star's recent Instagram clip -- she seems to be in pure bliss as she flashes the camera with her bare butt and enters the blue water surrounding the stunning island of Sardinia. She dives right under, and comes up with a huge grin across her face. Instagram / @maisie_williams

Her friends also got in on the cheeky carefree fun, posing together topless -- but covered by the water -- in one pic, and diving down to show off their bottomless behinds all sticking out of the water in another snap. Instagram / @maisie_williams

Maisie stayed at a guesthouse that offers yoga and a sauna during her trip, according to her IG post. It ap

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