Not only sulphur oxides, particulates as well (see my first quoted link). Particulate matter from combustion is extremely nasty. These floating petri dishes choke everything in their path.

> But those things add cost, and why would a cruise ship company do it if they are not compelled legally and their customers don’t mind?

Because their owners give a shit about the world they're leaving to their children?

Oh, well, I suppose they don't, then, do they?