Hello Skyglider, At the age of 14 I started work at my grandfathers full service glass shop. Now at the age of 43, and employed as a mobile auto-glass installer, I consider myself an expert with 29 years experience. The water stains on your windshield are from the minerals in ground water and are actually etched into the glass. They can not be "washed" off. They need to be polished out. Most glass shops will have products near the counter to remove water spots. They all consist of very fine pumice powder mixed with water to form a paste. It takes a lot of elbow grease to make it work, but it will. You may also find different types of acid for removing water stains but they are very caustic and I would'nt recommed them.
Rusty If You Want It Bad, You Get It Bad The Worse You Want It, The Worse You Get It