What Rickie said...
The only place I can think high voltage might accidentally get introduced to the unit ... in a boat... is if the motor charge voltage went wild .... a fuse at the battery would protect from that......There is going to be no way to protect anything if your boat gets struck by lightning.
I think a 3 amp fuse is recommended for the Helix 9. Use a quality spade type fuse ....The glass fuses can introduce voltage drop and intermittent problems over time.