Justin Baldoni Surfing in Costa Rica on Birthday Trip Amid Legal Mess, Check out Video

Things are be popping off in a big way between Blake Lively and Justin Baldoni in their ongoing legal drama ... but JB is keeping his cool, cruising some waves and celebrating his birthday in Costa Rica.

Check the pics -- Justin stripped down to his swim trunks, flashing those washboard abs with a surfboard in hand ... all while soaking up the sun and hitting the waves and making a splash for his 42nd birthday Sunday with the fam.

You’d never guess Justin's knee-deep in a serious legal mess ... the guy looked totally carefree, soaking up the beach life and riding those killer waves like he doesn't have a worry in the world.

Justin’s always been known for his love of surfing -- and he showed off some seriously solid skills -- but as the clip makes clear, there were a few wipeouts too ... a little symbolic of how the past two years have gone amid all that "It Ends With Us" legal drama.

Of course, more and more has been unraveling lately -- with court docs last week revealing a very damning text message that Taylor Swift sent Blake that straight-up called Baldoni a "bitch." The Blake-Baldoni trial is set to kick off May 18.