I am a Union electrician in NYC. The best advice I can give you for a 12volt system is to find out the correct wire size for the device you are using. The wire ampacity rating should never be smaller than the fuse you are using. If the device you are wiring draws 6 amps and has a 8 amp fuse, Use a conductor that is rated for a least 10 amps at 12 volts. Try not to exceed 80% of the wire rating. Usually I see wire conductors in boats that are around 10, 12, 14, and 16 AWG. Not including battery cable. AWG stands for American wire gauge, Every wire conductor has the type of wire and size stamped on the jacket. For instance [THHN 12 AWG] I do reccomend using marine wire for boats because the insulation is suitable for the conditions you will be in. Its also a good idea to coat all your connections with liquid electrical tape or a silicon based product. Heat shrinks work well too.
Know Before You GoLocal Weather | Marine Bouy Weather | Inshore Forecast | Offshore Forecast | Interactive Wind Charts | Tide Charts | Sea Surface Temps | Chlorophyll Concentrates | Online Chart Viewer