For this post, when I say 'engine', I'm referring to the ICE. 'Motor' is the electric...
That maximum RPM is the engine RPM. When setting a boat up from new, or with a new engine, we test run at Wide open and see what revs the engine gets to. If it's higher than the spec, we use a higher pitch prop (more forward movement per revolution), if too low, we put a lower pitch prop on. The aim is to get the engine to revs at or near the top of the limit. If you have too much pitch, that doesn't just affect the top end, it overloaded the motor through the whole rev range.
The drive with this engine would have been 1.98:1 (standard ratio supplied with the 120hp engine), so the prop speed would be almost half, about 2,000rpm...
Set up the electric motor, then take it out and see what RPM you get from the motor. I would aim to have it run around 5,000 to 5,500rpm at full 'throttle' (that's motor revs, not prop revs)... If it is too low, use a lower pitch prop (allow the motor to spin a bit quicker). With a petrol engine, the 'rule of thumb' is that each 2" of pitch will change the engine speed around 400rpm. But as I have no idea of the torque characteristics of the electric motor you're planning on using, that may work, or it may be out the window. You're blazing new ground here.
You could also utilize the water pump in the drive to cool a heat exchanger for the motor's oil...
The other consideration is how you deal with forward, neutral and reverse. I'm not sure how it's done in a car, but I know that synchronous AC motors are quite happy to spin 'backwards' . You can't do this with a drive. You need to have the motor spin only one way (CW looking at the 'pulley end'). You may want to bring the motor to very close to a stop as the drive comes out of gear, this should allow you to do away with any sort of shift interrupter. But you'll need to have the motor spinning, even if very slowly, as you go into gear (forward or reverse)...
Cheers,
Chris.......