There are many tales of persons getting into serious difficulties on large expanses of open water, due to an inability to start motors when their battery fails. So may people think that an old disused automotive battery is adequate for their needs, when in actual fact, they should be kitting their vessel out with a battery designed for marine use. At the minimum, a marine hybrid battery should be considered, they tend to have heavier plates than your typical auto batteries, and have better Cold Cranking Amps (CCA) specifications than car batteries. A true deep cycle marine battery is considerably more costly, but if that is the requirement for your application, then using anything less is at the least, foolhardy, and at worst, down right dangerous.