>What about the giant container ships with their engines idling all day whilst in port.
You are mistaken. By-and-large, no ship runs its main engine(s) in port. The exhaust you see from "giant container ships" (or most other ships) is their generator(s). The only exceptions are likely to be ferries with only short periods in port, their generators powered by their main engines, the ship's propulsion disconnected by a gearbox.