I bought a Fluke tone tracer for tracing out telephone lines in my house. It would work real well on a boat too, because you don't need to be able to see the wires as long as you get reasonably close, and signal will lead you right to the end of wire.
My house was wired by real fly-by-night. When I remodeled my kitchen I found two flying splices with wirenuts in the wall behind the counter, a plastered over junction box facing into the adjacent room, and another with burned up wiring buried in the ceiling! Hate to think what might be in the rest of the house.
Gary H. Lucas