"If you were born in international waters, what would your citizenship be?"?

You would be a citizen of what ever country your parents are from, and what ever country you most recently left.

When you turn 18, you will have to decide which country to be a citizen from.

Probably the nationality of the parents.

What your parents are, although conceivably, depending on the country, you might have a claim to citizenship in the country in which the ship is registered.

Pirate.

I’m not sure, but I think the US is one of a minority of countries that confers citizenship on anyone born within its borders. For most of the world, citizenship is based on parentage, as well as some other factors—like how long you’ve been in the country. There are even some nations of which you can never become citizens, not even if born there while your parents reside there.

You would be from Wales.