in my experience, actually those watermarks on glass either windscreen or windows are not typical watermarks
if the paint work has been wax treated during polishing.. it is actually the melted part of the wax from roof paintwork polish after a typical hot day followed by rain. wax melted and get carried by rain water and stain the glass.. they are hard to remove without polishing it off.
On my previous ride i had zymol (carnauba base wax) treated on the paint work. and i have tonnes of watermarks on all the glasses and a real eye sore. very difficult to polish off. i tried all kinds of polishes, finally i rid the car to rid the eye sore.
my current ride, i stayed away from wax polishing since day 1... till date after almost 3 years.. not a single water marks on the screens and windows.. same open parking space. I use nano coat type paint protection these days only.
if you wanna wax, dry it immediately after rain..or else stay away from those natural wax based polish.