Many really large ships tend to have diesel generators and electric motors. The reason for this is 3 part:
1. Diesel (and gas) engines have a much narrower band under which the engines are most efficient. By only running them in this narrow power band you gain range at sea and lower operating costs. Also a much smaller generator can usually be used, partially or fully offsetting the cost of the dual drive. Historically, this was called a diesel-electric drive system rather than a hybrid system.
2. You can build larger ships, because a ship that would take an unreasonable (possibly unfloatable) number of diesel engines to get underway (or giant tugs that may not be available) can be pushed by electric motors. This is due to motors having a basically flat torque curve (i.e. the same torque at all speeds.) HP for electric motors changes with speed just as in a diesel or gas engine.
3. The fuel for diesel engines stores much more compactly. Of course, if your power needs are lower you can design the electrical storage into the ship's ballast system.
Note: diesel and gas engines are rated at their maximum hp.
As for nuclear ships most use steam to turn the props, but many use electric motors.
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