Research on powering a narrowboat goes on apace. Information in now from various sources.
The prospective narrowboat will be gas free. i.e. there will be no gas appliances like cooker, heating. This means we will need a generator or a very large battery bank. There are gas free boats around (ever since Whisper) but these are diesel driven and have a generator either on the engine or in addition to the engine. Primary motive power is still the diesel engine.
My research has led me to the following options:

1.) Conventional diesel propulsion. Diesel Engine drives the propellor via propshaft.
2.) Diesel engine with hydraulic transmission. You can put the engine anywhere in the boat. The engine can be run at constant revs and prop speed is controlled via the hydraulics.
3.) Diesel engine with auxiliary electric drive. Both diesel and electric motors drive the propellor through the same propellor shaft using clutches and belts to engage one or the other. Thames Electric Launch selectric system.
4.) Electric motor drives the propellor using batteries charged from shore power or via an on-board generator. Solomon Technologies Electric Wheel for instance, from Enviroboat.

While I'd like to go for (4) I think it's too expensive (add 15k to the build cost of the narrowboat) and I'll probably go for (3).