Aveline I'm assuming you've just responded to my post without reading the link or doing any research yourself?
Think outside the box for a moment - from the mining and processing of raw materials to make the ships and everything in them; the the amount of diesel used; the food waste; the human waste; I could go on....instead you may like to read this, from Friends of the Earth, 14 March 2022:
"How Much Sewage Do Cruise Ships Dump?
More than a billion gallons of sewage are dumped into the ocean annually. This sewage is not only full of human waste, but also chemicals, pharmaceuticals, bacteria, viruses, heavy metals, and hazardous waste. And many cruise ships lack the sewage treatment facilities to adequately filter out toxins.
The increased waste in the ocean adds to the problem of pollution and oxygen depletion in the waters. Added nitrogen and phosphorus allow for algae blooms — which can suffocate coral reefs, manatees, shellfish, and fish.
Cruise ships often use cheap, dirty, heavy fuel to power their engines. And their smokestacks must be cleaned to remove the harmful elements from the exhaust. So cruise ships have implemented using scrubbers to “clean” their dirty fuel air emissions. This allows the cruise liners to continue to burn cheaper, more harmful fuel. And the water used in the “scrubber” process is then discharged into the ocean adding more pollution to the already polluted waters. These scrubbers emit acidic wastewater that is warmer than ambient sea water and contains heavy metals, polycyclic aromatic hydrocarbons (PAHs), suspended particulate matter, and nitrates, all of which are harmful to the marine environment."
I despair that we humans seem to be developing ostrich-like DNA