The difference between a marine battery and a car battery is simple. Marine batteries are designed to have the burst of power you need to start the engine of your boat. But it also has the capacity to power on board gear and accessories. They’re also more resilient to things like heavy vibrations and pounding waves. Car batteries basically just need that initial burst of power to start the engine. Then, the alternator takes over, over the course of the trip, and charges the battery.