Cruise ships dump waste in the ocean ?Gray water? ? the water that is collected from showers, sinks, and laundry facilities ? is also dumped into open ocean waters and graywater can be just as polluted as sewage but there are even fewer rules for where this can be dumped.

The primary concern with cruise ship water is not typically the drinking water on board—which is heavily treated and regulated—but the massive volume of wastewater (sewage and graywater) the ships produce and discharge. Large cruise ships are often described as "floating cities" that can generate over 200,000 gallons of sewage ("black water") and 1 million gallons of graywater (from sinks, laundry, and showers) in a single week. While modern ships use advanced treatment systems, international laws allow for the dumping of treated sewage anywhere and even raw sewage once the vessel is more than three miles from shore. This discharge can contain high levels of bacteria, nitrogen, phosphorus, and toxic chemicals that sicken marine life, destroy coral reefs, and lead to harmful algae blooms. Additionally, ballast water carried in the hull can introduce invasive species or pathogens like cholera into new ecosystems when flushed.