That is correct. However, the pollutants that the referenced ships produce are far more hazardous (immediately) and in greater volumes than many countries produce (in aggregate) per year. To put it in perspective...warning: this is an analogy that mixes metaphors:
Talking about CO2 emissions while Nuclear Waste is constantly being pumped into the ocean is shuffling the deck chairs.
Well, that change is already happening with the ongoing big change towards LNG as the preferred fuel for shipping.
But when it comes to reducing CO2 emissions, outside of improving energy efficiency, it seems nobody has anything even remotely viable today. That goes both for deep sea shipping and for most air travel.
The German company SkySails has been working for years on kites that can reduce fuel use on large ships. I think at some point we'll see alternative (renewable) energy taking over shipping as it is gradually taking over other domains.