I don't think they can switch to bunker. For one, most commercial ships are Diesel or gas turbine. The days of steam engines are mostly gone. The U.S. Navy combatants are now all gas turbine.

If one wants to place blame for emissions, look to air transport. I once saw a paper on the tonnage of emissions that a four-engine airliner spits out on a single cross-continent flight. The number was somewhere between 50 and 65 TONS and that did not count the particulates.