It's not a legal thing, that's for certain.

Welp, bombing the crap out of someone isn't necessarily "legal" either, so you're going to have to comfortable with the idea that nation-states ignore laws when it means defending themselves. Since you mentioned Maritime Law, what does it say in Concept about The Freedom of The Seas and those who attempt to deny it?

Generally speaking, when the US decides to blatantly violate international law, whether it's a "good thing" or not depends on whether the benefits of doing it are outweighed by the consequences of doing it.

So my calculus is that I'd rather have to have the State Department call up some government whose flagged ship was just impounded for carrying missiles to the Houthis vs. having to explain why I didn't do everything in my power to protect the sailors I put in harm's way.

But that doesn't give us the right to board cargo ships that aren't actually attacking us. If we were to declare war against Yemen and institute a blockade, we would have the right to do that....but we're not in that situation.

They're currently shooting Iranian weapons at us with pretty much impunity and have been for...8 or 9 months? That's a long time to roll the dice.

In some scenarios the best defense is...more offense.