electrical

[tsubaki]

There are many routes that can be taken toward rewiring a boat, it working correctly is just as important as you knowing how it is working in order to run down problems easier. I have never seen any two boats wired the same way, even from the factory.
Here's the best I can describe my take on it.

Keeping the hot, ground and signal wires from the engine to the gauges dedicated towards these is the first priority, don't be tempted to derive a hot or ground from the engines harness for non engine devices. This usually has a fuse/breaker/overload at the engine, don't want to overload it or create a situation to backfeed through a ground.
Stuff like running lights, instrument lights, fuel gauge, radio, depthfinder, bilge, etc need to be fed from the fuse panel through their appropriate switches and have a ground back to the panel, not looping between each other unless on the same fuse. Having an independent fuse for each device is overkill but it will rule out problems and prevent a total failure during an outing. Lets just say, if an instrument light shorts, it will not affect anything else.
Older gauges may not have the instrument light bulb ground (-) independent from the instrument gauge, this is something that may need addressing. Changing gauges may be an option, I can't answer just how much power capability any particular engines source can handle.
Normally the average fuse panel will not exceed the capability of a #10 wire being fed from the battery but most boaters feel the need to run #6 wire instead. Each boater will have differing circumstances and needs. You need to know what you intend to operate electrically and where in order to determine what is best for your situation. Some people prefer circuit breakers, some prefer different fuse types.

This is how I do most of my boats.
Each engine instrument gauge is fed from the engines power source and ground.
(Fuse 1) All instrument bulbs are fed from the panel through a switch (to reduce night blindness) and has it's common ground derived from the panel. Also the fuel gauge is connected to this.
(Fuse 2) Running lights are fed from the fuse panel through a switch and have it's common ground derived from the panel.
(Fuse 3) VHF Radio, has it's separate ground to the panel.
(Fuse 4) Depthfinder, has it's separate ground to the panel.
(Fuse 5) Horn, has it's separate ground.
I'm sure I left out others, you get the idea.

The connection for the bilge is automatic only, connected to the battery (fused).
The fuse panel is fed through a main switch at the helm and is fused at the battery connection.
Each plug or screw connection has had dielectric grease applied to it.

It's not that complicated, just first need to understand why you are doing what.