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Zac Efron and Nina Dobrev are sparking romance rumors ... joining two famous couples on a yacht trip after her split from Shaun White . Backgrid

Check out these photos of Zac and Nina onboard a floating paradise off the coast of Italy ... they're yachting with Miles Teller and his model wife Keleigh Sperry , plus Chace Crawford and his model girlfriend Kelsey Merritt .

Nina's newly single after her engagement to Shaun was called off ... so it's interesting she's traveling with an eligible bachelor in Zac and two other couples. Backgrid

Zac's rocking some platinum blonde hair and looking jacked ... dude's a stud.

Nina's a dime herself ... showing off her curves in some revealing black swimwear. The "Vampire Diaries

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