Cruise ship toilets don't dump directly into the ocean.
When you flush the toilet on a cruise ship the sewage does not go directly into the ocean but instead when you flush the toilet the sewage goes into a sewage tank and then goes through a sewage treatment facility on board the cruise ship.
The sewage on a cruise ship gets treated first and then the sewage gets dumped into the ocean.
Sewage can be dumped into the ocean from cruise ships but the sewage on the cruise ships must be treated before the sewage is allowed to be dumped into the ocean unless they are more than 3 miles off U.S. Shores.
U.S. law allows cruise ships to dump raw sewage in the ocean once a ship is more than three miles off U.S. shores.
Ships can dump treated sewage anywhere in the ocean except in Alaskan waters, where companies must comply with higher state standards.
However most ships treat the raw sewage all the time before dumping the sewage anywhere into the ocean.
Just like RV's cruise ships have holding tanks for the sewage and it's not directly piped into the ocean.
Cruise Ships also treat water from the ocean to use onboard the cruise ship for showers and other water needs.