Take Water Marks Off Vehicles

It's difficult to take water marks off vehicles once they dry up. The best way to avoid the problem is not to allow them to form in the first place.

Steps

Keep a clean, soft, dry cloth in your vehicle.
Wipe off any water marks as soon as they appear. This may seem obsessive, but it can really help your vehicle look nicer.
Use a cleaning agent. If you don't notice water marks until they have already dried, rinse the area with clean water, and wipe it off before it dries. For stubborn marks, soap or light detergent solutions will help.
Wash the car when its surface is cool to the touch. Wash normally and rinse well.
Consider washing again with diluted white vinegar. This will remove any wax and mineral deposits from finish and glass. Rinse. Follow with a good-grade polish and wax.

The use of grease-cutting dish soap such as Dawn will help remove road grime and water stains. It will also provide a barrier to prevent buildup in later washes. However, this can strip off wax that you'd prefer to keep. You may need to wax again.
Start at the top of the car and work your way down.
Never let soap or vinegar dry on the car.
As noted, it can be difficult to take water marks off once they dry. Get to them with a towel or other cloth while they are still wet.

Don't use anything harsh such as a Brillo or other scratch pad. That can ruin the paint.
Never use stain removers, spirits or any cleaning agent containing alcohol.
Never try to remove water spots from paint simply by applying force while wiping. That can damage the finish. Rely instead on the ability of a cleaning agent to gently loosen and remove the minerals in the marks.
Using grease-cutting dish soap or vinegar will remove all wax from your paint and leave it unprotected. Re-wax your car any time you wash it with dish soap. Instead you could find a milder car soap to protect the paint.