There actually is a law (rule) of large tonnage. However I doubt the ferry is big enough to qualify for that rule.

The ferry did take action. It reversed it's engines and was moving very slowly at time of impact. Those ferries cruise at 15 knots. At that speed the powerboat would have been destroyed with a strong possibility of fatalities. The powerboat took no action and apparently no one was on watch.

In an Admiralty court I expect that it would be a 90/10 decision - 90% of fault would go to the powerboat operator.