I insure my boat through Boat US.
They cover the boat and not the driver. Obviously as long as your operating within the law (not drunk etc) they will cover damage to the insured boat regardless of who you allow to operate it.
Forunately i have never had to file a claim with them. Based on what i have read on the internet about folks who have had to file claims with them they do a quick efficient job.
I do know that their rates were competitive for me.
I switched to them from progressive insurance after progressive's rates went up almost 250% in one year. This occurred when i moved from maryland to louisiana. Guess they think all this good saltwater fishing is gonna make me use my boat more - therby strongly increasing the likelyhood of damage..
You do have to maintain a boat us membership to get the INS... but at 15$ or so a year with a lot of perks i don't mind.
http://www.boatus.com/