Wladimir Klitschko -- STALKED IN THE OCEAN ... By Ex-Heavyweight Champ (Video)

A former heavyweight boxing champ who's OBSESSED with fighting Wladimir Klitschko has taken his pursuit to the sea ... chartering a boat to HARASS Klitschko this morning while he was out paddleboarding in Miami.

The video is maniacal -- showing Shannon Briggs hammering Klitschko with taunt after taunt ... while buzzing right by the guy ... eventually knocking him off his paddle board.

At one point during the incident, you can see and hear Klitschko tell Briggs, "You crossed the line."

Briggs has been all over Klitschko recently in an effort to coerce him into stepping back in the ring so they can fight.

Briggs recently found WK while he was eating lunch at an Italian restaurant and ignited a physical confrontation in which Briggs cut his foot on broken glass and ended up in the hospital.