I would believe that the hull is grounded anyway. The outboard motor mounts directly to the hull. In my opinion it would be better to have the hull grounded. No difference in electrical potential therefore no chance in any galvanic action. To clarify I would ground or bond the hull to the negative side of the battery. I would not use the hull as a ground path back to the battery. I would still run both positive and negative wire to each device being powered.
The love for fishing is one of the best gifts you can pass along