A marine propulsion system for a marine vessel includes an engine effectuating rotation of an output shaft, multiple batteries configured to power a marine vessel load, and an alternator having a rotor that is driven into rotation by the output shaft and that outputs a charge current to the batteries. The marine propulsion system further includes a control system configured to operate the engine in a propulsion mode and a generator mode. Upon receiving a generator mode command to start the engine in the generator mode, the control system operates the engine in accordance with a set of generator parameters to charge the batteries while a shift system of the marine propulsion system is locked in a neutral position such that the propulsor cannot be engaged and the marine vessel remains stationary.