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The one million number would seem to be an exaggeration at first, but a simple calculation gets you there. Assume a ship diesel produces 25 times the particulate produced by an automobile gasoline engine. I wouldn't be surprised if it is much worse than that because cars burn highly refined gasoline in optimized combustion processes that produce almost no particulate emissions. Cruise ships burn heavy fuel oil that is left over after the lighter components have been refined out and put out a lot of particulate emissions. Note the article is about particulate emissions, not all emissions.

On average, cruise ships use 140-150 tons of fuel per day. Source: How Much Fuel Does a Cruise Ship Use?

Measurement & Units	Value
Number cars	1,000,000
Miles per year avg	13,000
Miles per day avg	36
Miles per gallon avg	28
Gallons per day per car	1.29
Gallons per day 1M cars	1,290,000
Gasoline density lb/gal	6.07
Gasoline consumption lb/day	7,830,000
Gasoline consumption ton/day	3,915
Particulate Emissions Ratio, Ship Diesel:Car ICE	25
Equivalent Ship Diesel FO Consumption ton/day	157

Ship Diesel FO Consumption ton/day	140 - 150

(As you can see, I've got too much time on my hands ;>) )