Water spots form when water dries on the surface of your car and leaves behind solid residue from minerals or environmental contaminants. More malicious water spots might even contain corrosive elements that can etch paint or glass if not dealt with properly. If this etching process is permitted to progress, water can produce a crater, which will allow it to pool, further compounding the damage. Summer is prime time for spots because water evaporates so quickly on warm surfaces.
This is a companion discussion topic for the original entry at https://www.hagerty.com/articles-videos/articles/2013/12/02/water-spots