I have been having the same hard water problem as well. Those sucker water stains won't come off easily. Best solution so far is to use clay bar on the body, and CLR water mixture on the glass or vinegar on the glass. Works great.

Lynn

 

After you follow everyone elses solutions and have removed the spots. Invest in a Mr. Clean Autodry system to wash the car. In 2 years of using it...
1. I've never dried my car
2. I've never had any water spots, even on the windows.

Not sure of other peoples experience with the Autodry system, but it sue works for me and my black G.

Scott