First, whale hunting bad, whales good.
Second, "direct action" groups are terrorist organizations, just like ALF, the Taliban, ect. ect., just because this guy isn't doing much damage by comparison, it is what it is.
If I was Captain of a ship in international waters and ANYONE tried to board my vessel by force, I would take whatever action nessesary to prevent that, and dump the bodies overboard. I don't know what international law says on the subject, but I am guessing defending your boat against an attempt to board by force is permited.
Do I think Japan should stop whaling, yes I do. Do I support terrorism, I do not.