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

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.

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” star is giving us the textbook definition of post-breakup hotness here.

Zac and Nina seemed flirty … at one point, Zac had a phone in hand, snapping photos of Nina as she played in the water.

The yacht trip was packed with fun activities … snorkeling, swimming, jet skiing, paddle boarding, tanning … and the fellas even climbed to the top deck and jumped overboard.