Hard water leaves its mark on shower walls and glass shower enclosures in the form of crusty little white lime deposits that can be real difficult to remove.
One solution is sodium carbonate, which is the base for many cleaners and is the primary ingredient in washing soda. For a home remedy, try using vinegar either at full strength or diluted a bit with water. This natural acid will cut through hard water stains as well as soap scum in the bathroom.
Once the water stains have been removed, you can keep them away by applying a protective coating to the surfaces. Car wax works great for the tile and fiberglass surfaces. For glass shower doors try using a car windshield treatment to bead up the water so that next time the cleaning will be a breeze.