The only problem with an electric auxillary motor is that it would take a ton of power and I'm not sure the battery size/weight to get any meaningful runtime out of it would be at all practical.
One thing that I have seen that seems 'new' and might make an electric motor work are variable-speed diesel DC generators. They feed some kind of DC-DC converter/charge controller to provide a fixed DC voltage that can charge batteries or feed an inverter and could probably supply DC to the motor, too, although I haven't seen the converters for 48vdc.
Supposedly they're extremely efficient as they have the electrical generation built into the flywheel, so there's no mechanical losses from a belt or shaft driven generator. Because the charge controller is setup to convert a wide range of DC voltage to a fixed voltage, the engine can be run at varying speeds depending on electrical need, rather than requiring a fixed RPM required to generate AC power. Battery charging can happen at low speeds for improved fuel efficiency. I think they also enable the use of very small diesel engines, saving space.
At least this way you could have one IC engine that does both generation and could act as a power source for a motor. With enough battery, docking and exiting marinas could be done on battery alone. And you'd only need one IC engine for electrical power and auxillary propulsion.