Wladimir Klitschko -- Blasts Shannon Briggs ... Speedboat Stunt Wasn't Funny

Wladimir Klitschko says he wasn't laughing when Shannon Briggs chased him with his speedboat last year ... warning that someone needs to stop Shannon's wild antics before someone gets hurt.

Briggs (a former heavyweight champ) has been following Klitschko around the world -- from restaurants to news conferences to training sessions -- trying to goad the champion into a pay-per-view fight.

Most of Shannon's stunts are harmless and mostly based around yelling at WK ... but Wladimir says Briggs crossed the line when he chased him on the water last year.

"We need to save Shannon Briggs from Shannon Briggs," Klitschko said.

"We can laugh about it, but chasing me with speedboats while I was paddleboarding ... it's not funny. It's just ridiculously bad."

So far, no word back from Briggs ... yet.