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You will need to choose a high spec battery with a large reserve, because at low RPMS (below 1200-1400 RPMS) the engine consumes more power than the charging system puts out, so you will need to be able to power all of the Pumps and ECMS, etc.
Having a large reserve capacity ensures that during typical usage (including trolling, no-wake zones, etc.) you will not have a battery left so weak as to not run the pumps and ECMs, or start the motors.If you are on a budget then you could use the Deka group 27M. This is a starting battery that has sufficient reserve capacities: http://www.dcbattery.com/deka_8a27.html