I went with Boatus.com when my new boat came in. Good rate and had what I wanted in the coverage.
One thing some should look for is whether it's a boat policy or a yacht policy. A boat policy is sorta close to a cars and usually doesn't cover things like salvage and enviornmental clean up. This really comes in to play if you are docked on a deep lake like Cumberland. If the boat goes down, the Corp will require you to salvage it and clean it up and it's on your nickle even if it isn't your fault.
It probably isn't as big a deal on lakes that aren't as deep or for people who mostly tow.
In general, it's a good idea for everyone to go through the details of your policy and confirm the values are what you want.