Hey Old Snipe,
I have just been through a very similar process to that which you are contemplating. We had a nominal 400AH battery bank, but it quickly became evident the useable capacity was a lot less than that.
I just separated all of the batteries, checked their capacity using a simple voltage/load/timer combo. Identified the single
was totally kaput, so off to the scrap metal guys with that one. The remaining pair of 100AH Calcium batteries are both running at about 20AH useable each. Not great, but good enough for weekending for now. (Note, finding $20 battery charger
on a boat is probably NOT a good sign.)
After three abortive attempts at rewiring, gave up, gutted the whole lot and started again. SO glad I did. Once I pulled it all out, and threw out all the speaker wires in the lighting
circuits, pieces of wire with multiple screw cap junctions, bits of copper oxide with plastic coating masquerading as wire, then put all the good stuff in another pile, graded by size, I had less than one tenth of the number of useless cables
and had identified numerous points of serious potential failure. Now I know what EVERY wire in the boat does, instead of HOPING I knew. I know every wire is logically and functionally protected by the right sort of switch/fuse/circuit breaker.
It may seem like a lot at the start, but I think it is worth gutting it, on a simple boat like ours, it is just not that a daunting job and the peace of mind is priceless. I think each of my attempts at improving the wiring actually made me more aware of what I had not really dealt with.