How does this rank in the grand scheme of greenhouse gases? Not that it doesn't still matter, but efforts should be focused on climate change as a whole, not a single cherry-picked gas.
In terms of SO2 and NO2 pollutants, cars burn very clean fuel, very efficiently, with a catalytic converter to scrub their exhaust. Ships run on shitty bunker oil, which is just one step above combustible sludge on the purity scale... And aren't equipped with catalytic converters.
The thing is, even if you legislate that ships can't run on bunker oil in your territorial waters, they'll just run on it while at sea, and switch to a legislated fuel when they dock.