i deal with water spots like on a daily bases. i detail cars.. the best thing to use is wheel acid. wipes them rite off. DO NOT!! get it on your paint or buy buy clear coat. unless u know what you are doing. because you can use it to take water spots off of paint, but its tricky. gota keep a steady flow of water on it but for windows it can just set.
you can also buy stuff made to take off water spots. not as easy as acid but works.
get acid if you can find it. i buy from a dealer. but heres a link
use this with windex and steal wool takess ome time but very nice outcome.