Installing a second battery in your boat adds many extra hours away from land. Having a dedicated starting battery and a dedicated "house" battery to run extra appliances with the engine off means you can party anchored on the water for hours without risking killing your battery and not starting again. It is not difficult to install a second battery with a switch, or even a bank of "house" batteries, to give longer life or added power for more appliances once you understand the basic principles involved.
Install your battery switch in an easy to reach location near the battery installation. Mount the switch directly to the hull inside the engine compartment. Use the hardware that comes with the switch to insure proper connections.
Connect the positive (red) port from the engine battery to the number 1 position on the switch. Always use the same battery position, typically the port (left) side, to install the engine battery so you always know which dial position is your starting battery. Forgetting which battery is which increases your risk of running down your start up battery leaving you stranded in the middle of the water.
Connect the positive (red) port from the house battery to the number 2 position on the switch plate.
Connect the neutral (black) ports of both batteries to each other. Connect the neutral (black) port of the engine battery to the engine.
Connect the neutral (black) port of your house battery to the distribution panel on your boat.
Connect the circuit breaker to the power connection on the control switch, and connect the power connection to the engine.
Add an additional house battery to the bank by connecting the second battery's neutral port to the current house battery's positive port. This "series" of batteries will double the available amps of power but not the length of time you have for use.
Add an additional house battery to the bank and increase the length of time a charge will last by connecting the two in a parallel configuration. Connect the negative port of the existing house battery to the negative port of the addition and positive port to positive port as well for long battery life but not extra power.
Never substitute automotive parts or batteries for marine parts or batteries. Water and electricity are deadly combinations so the proper equipment is crucial for safety.