Honestly, so long as its in International waters, IDGAF.
A human life, IMO, is no more important than the life of a creature in an endangered species simply because they're human. If the Japanese want to be A-holes and hunt the whales to extinction, they should be ready to be attacked in turn by people who are sworn to protect the whales.
Of course, no country is going to take action. Attacking a Japanese whaling boat with one countries military would start a war, and there isn't a ton of reason to take the stupid activists into custody if its in International waters. Yeah, they attacked a Japanese whaling boat. My country should care because?
Let the Japanese deal with it. If they attack the boats within a nations waters, then sure - take them into custody. In international waters though, IMO its the problem of whichever country's boats were attacked, not the whole world's.
To be completely honest, I wish these guys would wise up and upgrade their ships to be more like Tiremes; get some decent ramming power behind those hits.
That and only target the bays that the whale is dragged up into. Disabling them means the ship will be unable to effectively hunt whales, yet it won't necessarily sink the ship either - dependent on the design.