Cruise ships are dumping an estimated $3.8 billion litres of (often untreated) sewage into the oceans each year, which is deadly to marine life. If you must cruise, take a look at Friends of the Environment's report card to choose a greener cruise line. See articles by The Guardian and Quartz outlining the problem. Cruise Control: A report on how cruise ships affect the marine environment by the Ocean Conservancy is a detailed look at the effects of this big business.