Not so sure about that. I think current trans-oceanic cargo ships generally run at fairly high fraction of peak power (otherwise they could have saved money buy buying a smaller engine in the first place). Also port operations tend to be pretty fast and efficient these days.

Of course, for ships running shorter distances and spending a larger fraction of their time in port loading and unloading, synfuels, hydrogen or for really short routes even batteries are probably going to be a better option.

Efficient in an economic sense. Nuclear plants are generally capital intensive, whereas fuel is very cheap. So you want to run them at close to max power as much as possible to recoup capital costs. But contrary to the previous poster, I think trans-oceanic ships generally do that.