I used to experience this regulalrly when landbased - boaties would come within 20-30 metres and often sever lines. A surf rod was kept ready, rigged with a 3oz bomb sinker on bright fluoro line, and after shouted warnings were unheeded, a big cast could put it across the bows.
Most never came within 100m again!
And for those who arced up...gaffs and big knives were brandished menacingly, so no altercation ever eventuated. Of course...this was when I was young and invincible! Being 6'4" and 250lbs didn't hurt, either!
Still, I reckon a good cast ahead of the bows still works well - particularly when they realize they are well within casting distance.