If your buddy is filing, he's probably working with an attorney. Might be a good time to schedule a meeting with the two of them, so the lawyer knows what's going on. Selling it for a ridiculously low price is a red flag, but if it was sold for a reasonable price (80% of current value of his half or something) it might be OK. That's a lawyer question, but I would think that since you're part owner, you're entitled to some sort of compensation because of the legal/binding agreement of shared ownership.
Just thinking out loud, but I wonder what would happen if you filed for a lien against the value of the boat, seeing as how you're legally a half-owner of it.
It might also be worth trying to get the DMV paperwork rolling, to help prove you legally have part ownership in the boat.
In any case, I'd strongly suggest talking to a lawyer. That's going to be the best chance of protecting your share in this boat. It'll probably cost $$ but that's nothing compared to the cost of losing the boat.