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Our diesel electric boats in the navy and the ferry systems don't have giant batteries for storing power, they just regulate the power to the electric motor. The first diesel electric boats in the navy might have been submarines and they needed batteries for running submerged, and I doubt the M3W will need that requirement, but in England one never knows :-)
The only battery necessary would be the engine starting batter which we already have in place.
As to electric efficiency, the efficient part is the engine running at a more constant rpm if you vary voltage with a big rheostat rather than varying engine rpm.
As Centa drive M3W owners note, a marine application is much more constant speed than a car.
Pretty sure a rheostat is an inefficient variable resister you would NOT want on an EV! But some other controller for sure
Nissan's
engine tech is a bit like you describe - engine drives generator drives motor. However they still put a battery in between the generator and the motor for better response, regeneration etc.
