Positive Ground Boat

I have what I guess is one of those rare boats with positive ground. It is a 1947 Matthews Deluxe 38. Although some documentation I have indicates a negative ground, the wiring placard mounted on the boat indicates positive ground. When I
first got the boat I attempted to change it to negative ground. This meant switching the battery ground wires, moving the port starter to the starboard engine and the starboard starter to the port engine and changing the starting and running connections to negative ground. THIS DIDN'T WORK. So now I am back to positive ground. My fuel tanks and pumps and water tank are grounded to the engines. I have an Onan MCCK genset that I want to install. I am redoing the AC electrics and want to install some electronic devices such a depth finder, GPS and radio. Most of these devices demand negative ground. The Genset must be grounded.

It seems like it could be so easy to accidently ground something that could create a direct short through some of my equipment and possibly fry some things. Is there a book that deals with this problem? Is there a way I can change my DC system to negative ground?