Your circuits should all be above ground, ie nothing earthed to the boat. What often happens in reality is that the engine
and hence prop shaft and prop are connected to the negative. That means that if any of your devices are leaking to earth you may get a current
path causing electrolysis
. To be on the safe isde, I would switch everything in the positive lead ( Its better to have a coherent system, anyway). We also isolate the engine
when it is not running, and switch off both sides of the circuit when we are not on the boat.(Important for a metal boat) Regards, Richard.