If you decide to rewire, where possible use the old wire to pull the new wire around and through cabinetry and bulkheads by tying or securing the new wire to the old wire and pulling through to where it goes. You can't do it for all your wires but it's a good tip when you can use it.
Also remove any old wiring where you can. I had to rewire all the gauges on a dashboard for a 30 footer, as an example, where the shop that installed the new gauges ran wires from the engine
compartment through the boat and up to the dashboard at the helm
. They completely screwed the job up so the owner took it to my shop for repair.
What I found was the other shop had simply cut the wires at the sending units and at the gauges and left the old wires in place. They did this for every device. It was a nightmare chasing down the gremlins in that boats. I had to make sure any loose wire hanging in that boat wasn't connected to any power source as well as attached to any device it no longer need to be attached to. You can't be at the helm
and be in the engine
compartment at the same time so troubleshooting an electrical
problem becomes exponentially harder down the road if you leave loose wires.