There is one good reason to install an extra battery in your boat: to keep you from being dead in the water. A dual battery enables you to turn one battery off and one battery on to power a stereo, refrigerator or lights at night--meanwhile preserving your starting side battery to get the boat going again when you are done.
Remove old battery. Clean the terminals if you intend on reusing it as one of your two batteries.
Place a second battery, along with the old one, or two new batteries side-by-side in the engine compartment and secure with straps, battery boxes, or bungee cords. Run marine-grade electrical wiring from the black (negative) terminal on the right battery to the black (negative) terminal on the left battery. Connect marine-quality electrical wiring to the black (negative) terminal on the left battery and connect the other end to the engine power.
Install a battery switch above the engine compartment in an easy to access location. The battery switch has four basic positions: "1" "2," "both," and "off." This allows you to switch between batteries, leave them both on while the engine is running to charge them, or turn them off completely so they don't accidentally drain when you are not on the boat.
Connect the red (positive) terminal of the right side battery to the No. 2 position on the battery switch. Connect the red (positive) terminal of the left battery to the No. 1 position on the battery switch.
Connect two marine-grade electrical wires to the "c" connection of the battery switch. Connect the end of one of the wires to the house circuit breaker. Connect the end of the other wire to the engine power. Connect a battery cable to the black (negative) terminal of the right battery and run it to the distribution panel.
Always wear protective gear when dealing with batteries. Protect your eyes with goggles and your arms with long sleeves. Buy marine-grade batteries for your boat. They withstand the pounding a boat takes and hold up longer than a car battery.