Put the HIN into a sealed envelope and mail it to yourself. The sealed, postmarked envelope can be presented as evidence to establish prior knowledge of the number. Take pictures as you build, and close ups as you install the HIN, keep one set at home for showing off, put the other set into the above envelope. You could probably even have the pictures and a certificate of origin of some type notarized if you're paranoid enough.

> If the boat is stolen and the thief gets a HIN for it, how do
> you prove it is yours?