A similar thing nearly happened to me today.
Happily leaving the harbour with a boat-load of passengers, when idiot on jetski/bike decides to accelerate across the channel without looking. On coming back into the harbour 35 minutes later, same idiot is floating around in the channel (so I slowed down) - as we approach, he decides to accelerate and then cut across right in front of me. When he finally sees me, he has such a shock he almost falls off. Luckily I had slowed down almost to a stop - so we drove round him. When we got back in, I found out that he had already been cautioned by the harbourmaster for being within the swim zone (yelloow buoys) along the beach.
While I don't neccesarily think that legislating people that drive boats/PWCs are licenced or should do some sort of test is the answer - but how do you get people to practice common sense?!? If I hadn't slowed down and hit him at any speed, the chances are he would be in a bad way (or dead) and I'd probably end up with a jail sentence (not to mention a concience)!!