Let me add a few things to the very good advice from Papyson. Having never bought a new boat in my lifetime, I've spent more than a few hours unraveling old boat electrical systems. Here are a few things I've learned...
* Whatever you find or do to that electrical system, document it and draw diagrams of what is connected to what. Get a box of tags-on-a-string-and tag each wire once you know what it feeds. In a pinch, masking tape will work, but eventually fades or comes loose.
* Invest in a good voltmeter/ampmeter that includes a resistance measurement function, which can also be used to test continuity. A diode tester is a plus.
* Spend the bucks to get different colored wire spools so that you can duplicate the original ones as best as possible. If you are re-wiring, which you will eventually do, you will need a lot of red, black, and green.
* Most devices use only 16 gauge wire, some use 14 gauge. However, I find that on a boat, especially in saltwater, all my new wires are 12 gauge or larger.
* I've always found that it is a good idea to run #4 gauge red and black wires directly from the battery to a distribution block under or near the console. From that block, you can feed everything you may need - voltmeter, console lights, running lights, fishfinder, GPS, radio, windless, spotlight, horn, etc. either directly or through a fused control panel. The 4 gauge wire will minimize voltage drop and can carry all the amps you will likely need. That also minimizes the number of wires you have in a long run from your console to the engine or battery.
* If possible, build a dedicated conduit between the motor, battery, and console in which to run your wires. Have holes or access points about every three feet or so to facilitate running new wires. The conduit protects the wiring, prevents kinking or pinching, and makes for a neat looking wiring system. I've used PVC, flexible conduit, and large split wiring covers at different times and places.
* Fuse everything, and document and mark which fuse is connected to which device. Pay attention to the power (amperage) requirements of each device, and use the appropriate fuse for that device.
* Whenever you make an electrical connection, make liberal use of liquid electrical tape. It minimizes corrosion at the connection.
* As for the trolling motor, it needs to be isolated on its own battery that can be charged by the motor through a switchable main battery switch.