In these situations, I always tip based on how I feel. It may sound stupid, but to me its emotional
A while back I had just bought a motorcycle and was trailering it home. I got ran off the road, motorcycle fell over in the trailer, and my vehicle was totaled. Tow truck guy came, made me feel ok about the situation, helped me right the motorcycle in the trailer, got my vehicle back on the road, etc. I was scared, the dude came and made everything ok. Hell yeah I tipped him well!
If you love that boat, and the driver alleviates all of your concerns, takes good care of it, and you can sense that, then tip him!
By the way, my vehicle was a jeep liberty. It was totalled, and still drove 300 miles home. I had no plastic anywhere remaining on my vehicle, could only open two doors, and one entire side of the car's sheet metal was partially cut away. Good vehicle!