You are thinking of a very specific type of pollution such as sulphur oxide. Ships have zero emission standards and burn very low grade fuel. Cars/jets etc have the sulphur removed from their fuel. However, this stuff tends to go away fairly quickly, in terms of long term pollution such as CO2 they are very low.

Don't forget Jet fuel takes a lot of processing and produces a lot of pollution before it's added to the aircraft.

Or more likely, the original source for that claim was thinking of a very specific type of pollution, but it then got badly reported and mangled so that a surprising number of people now think that a small number of ships produce a major chunk of all emissions.

As I recall, the article which really made a splash on this topic was clearly written about sulphur dioxide, but the headline just said "pollution" and everybody ran with it.