Just google "hard water car wash". I found thousands of responses. Here is one.
"I went down to Home Depot and picked up a 3-gallon commercial spray bottle. I filled it with distilled water, and I used the distilled water as a final rinse after rinsing the car with tap water. I used about 1 1/2 gallons (about $1.50 worth of water) and there was not one water spot on the whole car!"
http://www.nsxprime.com/forums/showthread.php?t=6928