Apart from being really stupid, his actions could also be construed as piracy. Being a maritime person (or so he would have us believe) 'ol Capt'n Pete should be aware that in many parts of the world, they haven't changed the maritime piracy laws in a good long while, hence the punishment for piracy in sovereign waters is sometimes something like "hanging till dead in the public square." (FYI: Piracy in Japanese waters can get you 5 years to life, or death if you killed someone during the act of piracy.)
The event occurred in international waters and Japan has a bit of a sticky issue here, due to their defence force caveats prohibiting use of force for anything other than national self defence. Fortunately for Japan, Pete made this easy for them by boarding a vessel that cannot be construed as anything other than Japanese, demanding money from what effectively amounts to the Japanese state, and interfering with Japan's national interests. Additionally, Im no expert on international law, but according to the UN Convention on the Law of the Sea, if the act of piracy occurs on the high seas, then ANY nation can seize said pirates and / or vessels and then try them subject to the laws of that country. Which brings us back to the 5 years to life. I think they have a solid case to try him at their leisure. If the piracy thing sticks, they also now have an argument for mobilising their "self-defence" force to protect their interests on the high seas.
And FYI: Japan's legal system has a 98% + conviction rate
Good one, Pete.