Sealant! Apply some, Park away from sprinklers. The best possible thing is to dry off your vehicle any time it gets wet.
But I had VERY good results with Klasse High Gloss Sealant Glaze 16.9 oz. I still washed my car weekly, but the water spots that were on my vehicle where gone after a simple wash. No corrections needed. Lasted for about 4 months, maybe a 2 week break from a wash every here and there. Just really enjoyed being able to spray the car off and watch the dry just slide away. Oh, and this was with just one layer of Klasse. You can apply a new layer every 24 hours, as long as the vehicle doesn't make contact with anything. Just makes me wonder how much longer this stuff would of lasted with 2 or three coats..