You need to cruise more.
The fact is it's costly to desalinate ocean water. That's why it's rarely done in the USA. If it were cheap, California wouldn't have a water shortage.
Next time you are on a cruise ask the environmental officer. RCI does absolutely buy municipal water. Plus water on the ship is chlorinated, you are just nose blind to the smell. I spent 40 years with a well, with no chlorine. I can smell it instantly.
Sorry, you don't agree, but what I posted is 100% true.
"seawater
Yes. All drinking water is either distilled from seawater or loaded onboard while the ship is in port. Royal Caribbean adheres to Vessel Sanitation Program standards published by the U.S. Public Health Service (USPHS)."