Water stains on fabrics are due to the detachment of the carving or finishing agents found on many garments. Water dislodges these agents and causes them to form rings on the surface of the material. To remove water stains, wet the entire area with water and let it dry. In addition to the methods mentioned above, you can use a clothes steamer filled with distilled water and white vinegar to wet the water stain before treatment.
Be careful to keep the steamer far enough away so that spray water doesn't splash onto the item. Simply re-washing clothes or bedding stained with water with your usual detergent on a normal cycle will often get rid of water stains. You can also hand wash a single garment several times with distilled water and a little detergent. Watermarks form when water stays on the fabric too long before it dries.
The best way to prevent them from happening is to speed up the drying time before they have time to develop. Here are some tips for cleaning fabrics so that watermarks stay with dirt. While sometimes annoying, the natural minerals that municipal systems add to the community's water supply are very valuable and provide essential minerals for purifying water and providing additional nutrients.