I did a quick calculation in my head and figured that if there around 300 cruise ships in service at any given time, then it would take around 3,500 to 4,000 cars to equal the daily pollution of one cruise ship. Does that sound about right?
See my table above. It works from the top to the bottom starting with 1 million cars and ends up comparing the particulate from those million cars to one cruise ship. The calc starts with an average number of miles driven per year and an average fuel economy (just educated guesses on my part). That gets you to 3,915 tons of gasoline burned per day for those million cars. I guesstimated that a car particulate output is 25 times higher than for a diesel propelled cruise ship which is the equivalent of 157 tons of residual oil burned on a cruise ship per day. That matches pretty closely to the value I found at the source cited - 140 to 150 tons of oil burned per day per cruise ship. My rough calc confirms that one million cars driven 36 miles each day produces the same amount of particulate as one average cruise ship.