If there were already ship engines produced at scale that were the same cost drop in replacements and could be used with zero-extra training by existing personnel, and were just as efficient, safe, etc. then I would of course want future ships to be 0 emissions.

Meanwhile, shipboard nuclear reactors have radioactive material. They can be cut open, so the radioactive material can be taken out. It can then be distributed throughout NYC by a terrorist with conventional explosives, killing millions.