Which would be great if co2 were all they emitted, burning bunker fuel is very dirty. Of all forms of emissions co2 is relatively harmless.

Beginning January 2020 ships worldwide are required to burn fuel with less than 0.5% sulphur or install scrubbers, so the problem has been greatly reduced. It's still way worse than regular diesel, but it's much better (about where diesel was at 30 years ago).