As far as this writer is aware, Cruise ship wastes are kept on board or possibly discharged far out at sea for no cost. To keep costs down for operators, certain marinas or nations could offer a barge based or mangrove/marina based bio-processing garden or off pump station. Using natural methods the waste food and fecal matter may be converted to clean water again.
Unfortunately, many of the stops of cruise ships are very pretty and susceptible to pollution therefore a barge that could remediate tank waste to higher or lower levels of perfection before being discharged to the sea may or may not be beneficial.
Thank you for considering this idea for implementation and entertainment.
Project artificial man groove or the bio pump LLC may be a corporate direction.