The problem is that you are assuming that what these ships are burning is [hydrocarbons] when it’s much more than just that. There are various impurities includ

The problem is that you are assuming that what these ships are burning is [hydrocarbons] when it’s much more than just that. There are various impurities including sulphur, which lead to extremely dirty emissions such as SO2. The OP is misinformed with regards to the “6 largest ships” claim and the “CO2” claim — as covered elsewhere in comments on this article.