Cruise ships range in size from a capacity of dozens of passengers, all the way up to well over six thousands people. That is the population of a small city.
All of those people have to live on a cruise ship for a few days, or even a week or longer. They expect nice conditions in well-kept staterooms where they take a shower and use a private toilet. These facilities use water, which may not seem like much individually, but it soon adds up.
So where does all that water go once it goes down the drain or toilet?