---

There are a couple types of wiring run on a boat,
First the connection from the Engine to it's starting battery
The engine if it is electronic start usually has a direct connection from the engine to the battery. There is a wire harness connection from the engine to the key switch which will usually have a purple wire for power to things like instruments (tach, fuel..) that you normally only want powered when the engine it running.
You then have the connection from from the other electronics (GPS, Radio...) to their power source. Depending on the number of items, distance from the battery or power source it's usually advisable to have a fuse panel, or really any distribution point where you can have one set of pos/neg feed from the battery feed multiple other end points.

The size of the wires required is a function of amps, distance and the expected voltage drop along the run. Here is a great calculator page that i use http://www.boat-project.com/tutorials...ulator.htm

Mitch, your accessory wiring sounds right. You are missing battery cables for the connection to the engine. If you remove the cowling, you should be able to find the starter and see the pos/neg connections You can get the part numbers by going to the online Merc parts catalog and reviewing the electrical components or calling your local dealer. Depending on where you plan to place the battery, you can also have a set of cables made

BSG, as a general rule, you want a pair of wires from the battery to a fuse panel and wires from the fuse panel to your accessories. The second set of wires is typically 16 gauge, the first depends on distance. Use the calculator to determine that based on where your battery is located.