The filthy fuel (called marine gas oil) means the big diesel engines on cruise ships emit tons of toxic smog that people on board and in port cities have to breathe. And while the smog is the most obvious in port, cruise ships also emit tons of greenhouse gases. (Cruising is actually worse for the climate than flying.

Cruise ships are significant polluters, though the industry is making aggressive strides toward sustainability in 2026. A single large cruise ship can emit as much sulfur dioxide as millions of cars, and the carbon footprint per passenger-kilometer is often much higher than that of a flight-plus-hotel vacation. Beyond air quality, ships generate massive amounts of sewage and graywater, which—while treated to international standards—is still discharged into the ocean. In 2026, the shift toward LNG (Liquefied Natural Gas) has reduced sulfur and particulate emissions by 99%, but "methane slip" remains a climate concern. Many ships are now equipped with "Scrubbers" to clean exhaust fumes and "Shore Power" connections to turn off engines while in port. While modern 2026 "Green" ships like the Icon-class or Silversea's Nova-class are the cleanest in history, the sheer scale of the industry means it remains a focal point for environmental scrutiny.