40 or more years ago it was quite common to find boats (and cars) with positive grounds. In other words everything was reversed. But today polarity is a very important issue. In other words maintaining positive and negative are hooked up correctly. So, now all boats are negative ground and switches and fuses are in the positive wire. I would switch it, but before I did, I would make sure polarity is maintained throughout the system. In other words positive is hooked to positive throughout.
to learn about DC systems see DC Electricity New Boatbuilders Home Page - Everything Boat Building - Basic Electricity DC Page 1