I don't think his lawful possession of the boat starts until that form was properly signed. Moral possession, sure.

Then whose boat was it, lawfully?

You raise an interesting point, by the way. America is partly founded on the idea that laws do not create rights, but instead recognize natural rights. If this story is accurate and the DHS agent did not recognize his natural rights, that is an offense. But if the law did not recognize his natural rights, that is an even greater offense.

I'm not sure - I think this is 'need a lawyer' territory.

But I would put forth the point that it's possible for an item to not be formally owned by anyone. Control is a kind of ownership, but it's not necessarily lawful ownership, otherwise a thief would lawfully own your car once he's driven off in it.

Perhaps it might be that he lawfully owns it, but can't lawfully control it until he gets the customs sign-off? But if he lawfully owned it while it's impounded, would that not mean he would have to pay licensing/registration for it while it sat there?

It does raise some interesting ideas.