On the contrary: ships are the third-best one when you consider greenhouse gas emissions per ton-kilometer of cargo moved, which is the metric that matters. They only pollute a lot as a category because there is so much fucking shipping going on. (Reducing that is also an issue, but one for a different thread.)

The only things better are bicycles and sailboats (because they use no fossil fuels at all). Even trains are less efficient, although in the long run they have the advantage of being possible to electrify and run on renewables.

Granted, the other pollution (not greenhouse gas) from ships is terrible because they use the cheapest, nastiest fuel. But as bad as that is, it’s still a much, much lower-priority concern than climate change.

(TBH, what we really need are nuclear cargo ships.)