Re: What size battery?
A car battery will be fine to start that little motor. The amount of bouncing around in the back of a boat is not that much different from under the hood of a car.
However one labeled "marine" might already have the wing nut terminals on it.
A marine battery is typically a deep cycle, which you don't need unless you will be running a trolling motor.... you need a starting battery. Both will work but you will probably pay more for a deep cycle or a battery labled "marine".