Water marks are ugly and can be considered a stain.

It is caused by washing with tap water, not religiously drying the car, allowing acid rain to settle. It is most clearly seen on your window panes, followed by those on the paintwork. Best seen when it almost glows when you wet wipe over it. IT IS ONE OF THE WORST STAINS TO REMOVE ON A CAR - period.

Also for light scratches on alloy wheels, duco, metals.