On my boat there is a "grounding block"... most marine applications have something like this under the helm area. You will see a bunch of smaller gauge black wires going to it and another larger gauge black wire going off to the engine ground. I would reccomend something like that instead of running the extra wire.
Same thing for the 12v supply, most boats have the "stereo" switch that powers up the stereo (and/or amps, eq... etc.) I have mine set up so i turn that knob and the amps, eq, hse... all gets powered up when I turn the switch. regardless of if the boat is running or not.
Hope this helps