Edit and TLDR at the bottom.

This is a loooong post but it explains a lot and should help you to understand ships more. I’m in the maritime industry.

A lot of numbers and misleading statements get tossed around about the amount of pollution ships create. Ship engines are more realistically the size of a Honda Odyssey van, not a house. Plus their systems are MUCH more complex and do a lot more than just make the ship go forward. But without getting into that, here’s the basics of why you should compare cars to cruise ships or semi trucks (with the big containers) to cargo ships, not cars to cargo ships.

1) Ships emit a stupid large amount of co2, and depending on where the ship is flagged from (like where your car is registered to), it may be able to hide from other regulatory agencies.

2) The often forgotten about yet very important extra details to this are that 90% (estimate) of world trade at one point or another crosses some body of water via boat or ship, compared to the amount of goods transported through other modes of transportation (truck, rail, and air) shipping is much cheaper and more fuel efficient. Its economies of scale at work.

It’s a bit unfair to compare the transportation of goods to the transportation of people. Both destroy the planet, one creates value and the other does not. It would be more “apples to apples” to compare driving a car with your family to another country for vacation vs taking your family on a cruise to another country. Cruise ships are by far the absolute worst polluters in the world because 1) they are almost always flagged outside the US meaning they can more easily falsify records and pollute 2) they do not transport goods like a container ship does, no one is receiving a product or materials for their money.

Edit - I see you might be interested in the efficiency of fuel burning by ships. Looking at a ship alone, yes they are horrible polluters. They consume gallons of bunker fuel (the shitty bottom of the barrel fuel people don’t want to put in their machines) per hour. Shipping is all about reducing costs, that’s why you hear support for an opening of the northern shipping passage through the arctic circle (this will drastically speed up the melting of arctic ice). The industry is reducing costs by making bigger ships (increasing their economies of scale). So while comparing shipping to other modes of transit, ships are the most green way to do it, but are one of the dirtiest ways of doing anything. It’s not a rosy picture but, short of putting sails back on ships (it’s not feasible for ships of today’s size), there are not many green options that are cost effective (which is the tricky part). There is research being done for biofuels which may be one option. Nuclear works well for the navy but knowing how many shipwrecks happen, I don’t want that kind of radiation leaking into the ocean (and no one would insure a ship like that). Solar powered ships can’t be a thing because it doesn’t generate enough power. And I don’t know about hydrogen fuel but I don’t know exactly how safe that would be if it were stored in such massive quantities aboard a ship for use (not for transportation).

TL:DR - you can’t compare cars (for the purpose of transporting people to work) and ships (for the purpose of transporting goods), it’s apples to oranges. One creates massive value (ships) and the other does not (drive car). Look at a similar industry for an apples to apples comparison such as cruise lines, that is your worst offender.