That’s not even the worst part of a voyage for shipping pollution. Environmental requirements in economic zones near countries often require ships to use more standard diesel fuel. Once in international waters though, they will switch from diesel to bunker fuel, since it’s cheaper and a majority of their fuel will be used in international waters without those restrictions.

A 2009 study showed a single container ship running bunker fuel pollutes about as much as 50 million cars.

https://www.theguardian.com/environment/2009/apr/09/shipping-pollution

That being said, that’s on a global scale. The congestion taxes are specifically about vehicle air pollution in busy congested areas. Those are not the same areas that shipping will be the biggest contributor.