I know mine is covered if I roll it off of the trailer, so I would like to think that rolling it in the water should be a "no-brainer"!
I suspect that they could pay you the lesser amount of either writing the boat off at it's depreciated value versus the cost of repair.
When I acquired my boat, I argued to the prospective insurers that it was one of a kind (in the used market) and they agreed to a replacement cost policy, at increased premium...