Justin Baldoni is focusing on good vibes only ... despite his ongoing legal war against his "It Ends With Us" costar, Blake Lively.

Check it out ... the actor is all smiles after hitting the ocean in Maui, Hawaii, beaming and carrying a big green surfboard while on vacation with his family.

Though, it's lookin' like Justin has swapped family time for some guy time.

We're told Justin joined a group of dudes -- including a couple surf instructors and another companion -- for a lesson at Ukumehame Beach Park Monday ... where he hung out for a bit before taking off.

Justin seems totally relaxed after his dip in the Pacific Ocean ... which is the opposite of how we'd expect him to be given his current drama with Blake.

As you know, Blake filed a legal complaint against Justin -- who also directed the controversial movie -- with the California Civil Rights Department ... accusing JB of sexually harassing her and coordinating a smear campaign against her, too. She made similar accusations in a federal lawsuit ... which Justin responded to with a couple lawsuits of his own.

The costars' legal teams are even battling it out over alleged evidence ... with both camps saying one behind-the-scenes clip proves their respective narratives to be true.

It's safe to say things are tense right now ... but you can't tell by Justin's sunny demeanor.

Surf's up, dude!!!