But I don't want to focus on airplanes here, but shipping, as the thread title might indicate. Not only ships burn low grade cheap fuel full of cancer and asthma causing pollutants, such as sulphur oxides, but they do so on a very, very inefficient fashion. And they aren't even fast as airplanes to compensate. Indeed, the ~15 largest cargo vessels, alone, emit more of those polutants over a year than the combined car fleet of the entire world (close to 800 million vehicles nowadays).
Now, you might argue those ships transport a fuckton of containers - over 500,000 square meters of cargo for the largest ones, to be precise - so the comparison isn't exactly fair. However, the combined gross tonnage of the ~15 largest ships put together, and divided by the combined car fleet in the world, would equal a tiny volume for every car - no larger than a suitcase indeed. And I doubt carrying a packed suitcase over a year on your vehicle would double its emissions during this period, so yeah.
Why are you jumping between an airplane comparison and a "car fleet" comparison? Your car fleet is no option at all for transporting goods between continents... with a lot of water between them.
And you need a fleet of airplanes to transport the content of one container ship... it is also much less efficient and bad for the environment.