It's a good question, but my knowledge of law in international waters is only second-hand. I've read somewhere that the laws of a ship in international waters was in someway related to the country where it's registered, but I have never researched the point, and I'm sure there's more to it. I would guess there's a whole separate body of law based on treaties between countries. But google could probably answer better than me on this topic right now.
But, if you're interested in this topic, you might find this article interesting: https://steemit.com/beos/@blocktrades/jurisdictional-agility-in-beos