They didn't stole the boat, they seized it. Not wanting to sign an erroneous legal document is respectable, calling the DHS stealers is clearly not. Don't be surprize if they won't cooperate to solve the matter ASAP.
Hence, they stole his boat. If the President were to show up at my parents' doorstep without a warrant or probable cause and shoot my little sister because he was having a bad day, would you call it a lawful execution? No, it's murder.
That's an extreme exaggeration to prove my point. But that's where your logic ends up. Having a badge that reads "Government Agent" does not mean all your actions are moral or legal.
He will get his boat back once the paperwork is completed correctly. Shocking, I know, but he will.
You think it's right for for a gov't agency to be uncooperative in retaliation for some perceived insult?