[dt5702]

The entirety of the drive train is huge. You have to turn two propellers which have to propel a ship weighing thousands of tonnes. To produce the necessary torque you need to have some big components. The supporting infrastructure you need to heat and circulate the fuel, then to supply it to the engines is another thing too. Gas turbines are much smaller but would be unsuitable for this particular application as they have good efficiency only at high loads. The range of loads this is used for would mean they are unsuitable.

[walrus01]

A number of very large cruise ships now have the drive entirely decoupled from the fuel burning engines. Same idea as a diesel electric locomotive. Big engine burns heavy fuel oil, turns generator, power goes to electric azipods.

[fulafel]

Smaller new vessels do this too.