Japan may also argue the boarding of the Shonan Maru No 2 was an act of piracy. Under the convention, Japan would need to show the boarding was a case of:
illegal acts of violence or detention, or any act of depredation, committed for private ends by the crew or the passengers of a private ship or a private aircraft, and directed […] on the high seas, against another ship or aircraft, or against persons or property on board such ship.
The sticking point for Japan may be whether or not the boarding can be considered to be for “private ends”.
I don’t see how that’s going to be an issue for Japan. I mean, that text isn’t going to be talking about personal versus organizational or broader ideological ends, if that’s what the author is thinking here. It’s going to be talking about whether he’s acting on behalf of a government or not. It’s to separate things like state actors from non-state actors. I am confident that no government is going to assert that he was acting on their direction in doing that: he wasn’t acting as part of a country’s navy.