I should mention that Diesel-Electric locomotion isn't unique to watergoing vessels. Most diesel locomotives that you see pulling freight trains are diesel-electric. Also, the giant dump trucks you see at mine sites are usually D-E.

DE is ideal for these types of applications. It brings the long distance and long run-time capabilities of a gasoline-fueled internal combustion engine, with the simplicity of an all-electric car.

A diesel engine is much simpler than a gasoline/petrol engine, and provides more torque, while burning a fluid that is less explosive. The diesel engine spins an electric generator. The engine can be brought up to its most efficient RPM range, and left there. The engine is never expected to perform outside of its powerband.

The electricity is then sent to individual wheel motors. There is no need for any gearing or transmission parts.

As an interesting aside, the electric motors are run backwards to provide braking for these behemoths. In the case of the giant dump trucks, the resulting energy is just fed through an enormous rack of heater-wire, that glows like a toaster while it expends the excess speed as heat. The trucks actually do have standard brake pads, but they are usually only good for an emergency, and if you use them while moving any faster than about 15mph, then you instantly burn them up and they must be replaced.