“Turning a screw outside a ship”
And the above statement refers to ????
Nuclear in any of the systems referenced do not replace the water, they replace the solar power used to heat the water.
You original comment stated:
Our modern navies seem to think, for large ocean going vessels - aircraft carriers - and vessels with trips that last a very long time - strategic submarines, that nuclear power is the best? Why would space travel be any different?
The reason space travel is different there is no medium available to exert force against. You have to take that medium with you. Most proposed systems use hydrogen or water for space travel believing those materials are likely to be replaceable in space travel.