Isn't the problem that ships are burning heavy oil instead of diesel or similar? CO2 isn't the main worry afaik, it's the rest that gets blasted unfiltered into the atmosphere and the left over sludge that gets illegally dumped into the sea.
https://en.wikipedia.org/wiki/MARPOL_73/78#IMO_2020
https://www.imo.org/fr/MediaCentre/HotTopics/Pages/Sulphur-2...
https://www.imo.org/en/MediaCentre/PressBriefings/pages/02-I...
The upper limit of the sulphur content of ships' fuel oil was reduced to 0.5% (from 3.5% previously) - under the so-called "IMO 2020" regulation prescribed in the MARPOL Convention. This significantly reduces the amount of sulphur oxide emanating from ships.