https://phys.org/news/2018-12-cargo-ships-emitting-boatloads...
Nothing is going to matter until this changes.
Cargo ships are responsible for ~3% of global CO2 emissions. Not too bad for providing a backbone for the world's economy.
"One giant container ship can emit almost the same amount of cancer and asthma-causing chemicals as 50m cars, study finds ": https://www.theguardian.com/environment/2009/apr/09/shipping...
Cargo ships are bad, much worse, than people think. And this has nothing to do with efficiency, or price, or anything else; pollution is pollution, and emissions != CO2.
This is an extremely reductive and absolutist way of thinking.
We have very real solutions to reduce emissions of cars with greener options, solutions that do not work with cargo ships.
Nuclear civilian cargo ships have quite disastrous potential considering the scale at which that would need to be introduced: More than 50.000 merchant ships would then be swimming nuclear reactors, good luck finding anybody willing to insure that.
Not to mention that it would require building a global nuclear fission centered infrastructure to support it; Expensive, and even more potential for catastrophic accidents, even more issues with where to store the waste.