I was intrigued - seems it could be because ships use
heavy oil
https://www.quora.com/Is-it-true-that-the-15-biggest-ships-i...
https://www.quora.com/Is-it-true-that-the-15-biggest-ships-i...
Thanks for the link. So it isn't carbon emissions that exceed those of cars but SOx and NOx (which are still a problem).