Electric motors for boats are becoming popular.
I imagine one of the difficulties is keeping the electric motor sealed on the rotating shaft so water can't get in.
I propose that the stator of the motor is sealed in a dry area, and the rotor is outside in the water.
It is important to minimize
the separation between the stator and rotor. The stator is keep in a strong thin plastic that radially surrounds the rotor.
My favorite type of electric motor is switched reluctance motor, which would be perfect for this application since the rotor is so simple: just a chunk of metal with arms radiating for the centre. The metal would be a rust-proof such as stainless steel. The rotor would be removable so could be easily cleaned.
Anyway, the idea is that the motor wouldn't need waterproof sealing (between moving parts) so could be cheaper, more efficient, etc.