Whatever diagram there was, is likely wrong due to whatever the P.O.s did to it.
One of the PO's of my boat rewired the engine wiring to remove the 'trailer plug.' That was a good thing. They left all the old wire, which was not a good thing. I removed all the dead wiring to simplify debugging. The PO had also run some real nice 120v stuff with romex and wirenuts. I removed that too.
I completely rewired my boat except for the wiring that's embedded in the deck for the lighting. New panels, battery fuses, light fixtures, all new 110 (with GFCIs even...), etc...
It really depends on what you wish to accomplish and how bad the current wiring is. A diagram is not all that helpful - it's basically common sense.