The MotoMaster inverter I linked to can be wired directly to the electrical system.
You can either wire it direct to the battery (preferably with a fuse) or into the ACC or IGN circuits of the boat.
Although the one I linked to will work, you may want to consider something marine-rated if it is a permanent installation. I don't know how insurance works for boats but if there was ever a problem with it and there was a fire, etc... using non-marine rated components might be a problem.
You could talk to the guys down at West Marine and see what they suggest, but it will be considerably more expensive.