I feel your frustration and I have eliminated water spots but piggy backing off of what jadnashuanh suggested.
1. Dont wash car in sun, period.
2. Use a proper car wash product.
3. Dry car completely when finished. I keep the car wet, I wash my F10 in 5 segments but after each segment is complete I rinse the previous segment, as soon as I am finished washing the car I dry with a good drying cloth. I am a big fan of Swissvax and its products, the towel I use that performs amazing can be found on amazon.com
http://www.amazon.com/Swissvax-SE109.../dp/B006D9EBDO. Be sure to use towel wet!
4. Lastly, keep in the garage when not driving, always.
Hope this helps some, it has worked for me.