What a water spot really is
When a droplet dries on your paint, the water leaves and everything dissolved in it does not. North Texas municipal water carries a meaningful load of calcium and magnesium, so what stays behind is a tiny ring of mineral bonded to your clear coat.
In July, with a hood at 140°F, that droplet does not run off — it flash-dries in place. That is why a driveway rinse at 3pm can leave your car looking worse than before.
Etching is the expensive version
Leave those minerals in full sun long enough and they stop sitting on the paint and start etching into it. At that point a wash will not remove them; it takes polishing, which means removing a layer of clear coat.
The prevention is trivial and the cure is not, which is the whole argument for a filtered final rinse.
Spot-free rinse, every tier
Our last rinse is filtered soft water with the minerals pulled out, so it dries clear instead of chalky. It is included on the $9 Basic, not sold as an upgrade, because a wash that leaves spots is not a finished wash.
If you do rinse at home, do it early morning or after sunset, work in shade, and dry with a clean microfiber towel rather than letting it air-dry.