I just had this happen to my car....twice in a seven day period! My situation was I at a house that uses well water, sprinklers kicked on and completely destroyed half my car, the second time and for good measure it hit the other side the next week!
I also think the heat of the day played a huge roll into the water spots, 90+ degrees...actual feel like temp was over 100 and there was not a cloud in the sky so my car was just baking in the sun. My sprinklers at home have hit my car and my wifes car before but never had any water spots...baked into the paint!
Prior to the first event, the car had WDGPS as the LSP that wasnt that old, the second time had a fresh coat of WDGPS that was less than 24 hours old!! The side panel that were exposed to the sun we spotted just like the hood...on the second event, the side panels were not exposed to the sunlight so they were fine.
Yes, I'm still pissed!