Re: Master switch doesn't work, thinking of rewiring the boat
First go buy a test light, or better a $25 multimeter. Make sure the battery isn't just dead.
While you're out try to pick up a copy of "the 12v bible" or "the 12v doctor's handbook" or a similar guide to 12v power systems for boaters. Easier to get a picture of the whole thing that way, than to get bits and pieces online. And not very expensive.