Per the linked Guardian article from 2009, shipping is responsible for 18-30% of all the world's nitrogen oxide (NOx) pollution and 9% of the global sulphur oxide (SOx) pollution.
Yeah, technically those are indirectly greenhouse gases, but just to note, the article linked points out that "Maritime transport accounts for 3.5% to 4%".
It's double-digit percentages of some pollutants that are very bad for human health, but relatively minor in terms of climate change.
EDIT: I take it back. NOx is significant. SOx not so much.
https://www.theguardian.com/environment/2009/apr/09/shipping...
Absolute numbers here: https://sci-hub.hkvisa.net/10.1016/j.scitotenv.2009.07.037