For OTC, Glass Science's Glass Scrub worked on water spots that were a bit more than outlines (not clouded through out). It may work on more extreme spots, but I haven't had any occasion to try it on such.
I don't know if it's very readily available anymore, but Strip It (Pylon) Windshield Cleaning Compound worked very well on hard (lots of rust) water coated glass. I used it on glass that hadn't been cleaned (other than when it rained) for over 10 years. Rusty haze that looked like it was nicotine coated, but it was on the outside! Worked quickly and easily. Not much scrubbing. It came in a black "toothpaste" tube.
Also, chrome polish is supposed to be good on glass, but I haven't tried it on water spots.