for an outboard boat, you will need volt, tachometer and fuel level as neccesities, you can add trim gauge if your trim sender works, and temp if you motor has a sender. All these wires should be in the engine harness. For the boat, run two #8 or #10 wires from the battery switch( red pos) and from teh negative battery post(black -) to a distribution panel(fuse panel), then feed your individual circuts from there. I tend to run duplex wire(pacer tinned wire) to each circut(on a scratch rewire), that way I have a supplied ground to each circut rather than using a common ground wire thats spliced adn pigtailed all ove the place. Makes it a whole lot easier to diagnos an electrical problem later. Get a fine line permanet marker and you can label the duplex wires for identification. You can use a label maker to add tags on the ends(then tend to get puled off when running the wires). If you have any questions, go by Power Boat Service on Atlantic Ave. Ask for Diego(tell him David sent you), he can give you some good tips and sell you parts if needed. Make sure to offer to pay him for his time, its worth it.
Skools Out posted some good links awhile back that had the corect color codes and wiring requiremnts, i think thses are the ones.
http://www.uscgboating.org/regulatio...gulations.aspx
http://www.ftkltd.com/Boat%20Wiring%...or%20Codes.htm