Scrub white vinegar on the water calcification stains with a scrub brush. Then add some water to the brush, and scrub again.
Make a paste with vinegar and baking soda. Scrub the paste on the water calcification stains, and let the paste set for 15 minutes. Then wash away the paste with water and your scrub brush.
Apply lemon juice to the calcification stains and scrub. If the water stains are not too bad, sometimes using just the lemon juice will work to get rid of them.
Scrub the water stains with a mixture of diluted bleach -- one part bleach to 10 parts water. Scrub the calcified area, rinse the area well, and dry it with a towel. Make sure you clean up any remaining bleach, because if you lean up against the area later you may end up with bleach stains on your clothes. Bleach stains are permanent.
You can also try a store-bought product that is made specifically for removing lime or water calcification build-up. Follow the product directions.