I'd agree with Toughluck. Boats depreciate rapidly and when you factor in operating expenses, I'd let it go. I'd be surprised if a dealer could sell it for $9K.
But, I've seen more than a few people keep small boats and jet skis in a Chapter 7 because the equity was small enough to fit into the state wildcard exemption or, more was owed than the boat was worth. Forcing them to surrender the boat will only create one more unsecured debt. It would be different if the boat was more expensive and clearly a luxury craft. I think they've a reasonable shot at keeping it.