This is the problem with lumping all pollution together: NOx is harmful to humans and animals in the short term, causing lung irritation. So yes, you really don

This is the problem with lumping all pollution together: NOx is harmful to humans and animals in the short term, causing lung irritation. So yes, you really don't want a bunch of ships pumping NOx near human populations. I don't know if a ships funnel would be enough to get the fumes away from the crew of a ship while it's running in the middle of the ocean. Excess CO2 is not especially harmful to humans in the short term (unless you are in a confined area), but it does contribute to global climate change.