You really think you'll find somebody that for a small insurance premium will fix anything wrong with your boat? Really?
My dad's camera went overboard last weekend. He was so glad he got the extended warranty. I was skeptical but didn't say anything. Turns out water damage is not covered.
Extended Warranties exist to make the companies that offer them money. Which is why water damage was not covered for my dad's camera, and why if you found a policy worth anything it would probably be cost prohibitive.
You might try contacting the mfg of the boat and see if they can offer you anything... but I doubt they'd cover everything on board... so you'd end up with a policy for the hull, another for the motor, appliances, etc, etc.