I have read that when the cruise ships of Celebrity and RCCL buy extra water from ports and segregate it, they then mostly use it for non-drinking purposes like laundry, cleaning, etc. There was a cruise critic article on this a couple of years ago.
I have always found the purified tap water on board to be fine.
However it does have some residual salts added so I do not use it in my CPAP machine. I ask for distilled and they easily accommodate that request.