I don't know how serious the water marks are on your windshield but on my car, this one normally works. Dilute a small amount of car shampoo with water. Wet thouroughly your windshield. Use a high quality chamois leather to wipe it dry using circular motion. Someone suggested using diluted vinegar in previous threads. Just be very careful with this, though, as vinegar is acidic and will definitely ruin your paintwork. Besides, acids also eat away the rubber around your windscreen. So, konting ingat lang.

Chamois leather can also be used to dry your car just after washing to leave the paintwork of your car "streak-free". Try it and you will see the difference. Just don't use the inferior types chamois leather (plasticky, rubberry ones).