Maby they know better than to mess round with rapidly spinning screws? If so, firing up the auxiliary might be a good way to discourage further attacks.

And why just in this limited area? Are these one rogue pod, or is the behavior spreading to others. Maybe they are responding to some real or perceived human attack in the past? Maybe the whales are pissed off about something they think the boats are responsible for, like military sonar activity or declining fish stocks. Or maybe its just a thug whale.

Orcas are cute and smart, but there's no reason they can't be bullies too. I know they like to kill penguins, seals and porpoises just for fun, not just to eat. They seem to be one of the few wild animals (besides men) that enjoys killing for sport.
Perhaps there is some new kind of sonic depth finder coming into recent use on yachts they find really annoying. I must admit I'm baffled.