Boat wiring and Boat Trailer Wiring is easy to understand. Think about this: How many ends or poles are on a typical battery? There are two: one is positive and the other is negative. For any electrical accessory there has to be two wires running to that accessory (Red for Positive, and Black for Negative). These must each be connected directly or through a fuse box to the positive and negative terminal of a 12 volt battery. If not connecting through an existing fuse box, you should consider placing an in line fuse on the positive side to prevent damage to your accessory. Most devices will tell you in their instructions about the size of fuse needed, in fact many will actually provide that in line fuse ready to install.
Boats usually have a built in fuse box connected to the positive and negative battery terminal. In many freshwater fishing boats this fuse box is connected to the starter battery. It is possible to add devices/accessories by tapping into this fuse box for power. However, you can run the risk of demanding more sustained power from the starter battery than it was designed to handle. This is especially true if you plan to use these accessories when the main motor is not running which means you will not have the advantage of the motors alternator powering your equipment. This is the time to begin to think about adding an additional deep cycle battery for your accessories.
The number one reason fishing boats will install deep cycle batteries is to run their trolling motors - these can demand up to 3 twelve volt deep cycle batteries. These motors draw lots of power, and are consistently used when the main motor is turned off. In other cases depending on additional accessories, you may consider adding an additional deep cycle battery to power GPS/Sonar unit, VHF radio, power wash down pump, radio, and an additional 12 volt jack to power a fillet knife for example.