Yep, boat wire is expensive. If bought off the shelf at the yacht store in small quantities it's really expensive. It's not a good idea to use any part of your boat as a ground conductor. It's hard to get away from using the engine block to ground the starter and some instruments. Unlike a car, other stuff needs to have insulated wire connection to the neg batt terms. Keep in mind a main breaker should be close to your pos batt term, prior to the wire run to your switch.
Fair winds and following seas!