[kbolino]

I think most of the heat and fuel will get wasted anyway. A nuclear-powered cargo ship is going to be closer to a nuclear-powered submarine than a nuclear-powered aircraft carrier, in terms of the number of people being supported and the needs beyond propulsion. That means running the reactor below its optimal fuel efficiency, and then intentionally discarding a portion of its heat most of the time to retain some spare capacity for demand spikes and emergencies. Of course, small-scale nuclear reactors are an active area of research, but AFAIK the results haven't been promising yet. So it's an avenue, but not the only one.

Running a reactor full-bore on shore, pumping ocean water, desalinizing it, electrolyzing it to hydrogen, cryogenically separating nitrogen from air, and combining the result to produce ammonia, might look really inefficient in isolation, but might be efficient enough when compared against the alternative. When the costs of logistics, personnel, and capital are accounted for, it might even come out ahead. That having been said, the amount of waste oxygen this would produce is immense, and could create risks of its own. Ammonia is also more likely to produce NOx when combusted in air than hydrogen is, so that's not great, either.