Bruce Springsteen is The Boss wherever he goes ... and that especially applies when he's soaking up some sun aboard the most popular yacht in celebdom.

Bruce was seen lounging Wednesday on David Geffen's yacht off the coast of Ibiza -- with his wife, Patti Scialfa, hanging out nearby in a robe. Calvin Klein was also there with his model bf, Kevin Baker.

Bruce started out in white swim trunks, but then swapped 'em for a darker pair ... which he eventually yanked up to catch some rays on his thighs.

Whoah ohhh ... he's on fire.