I'd suggest clay barring the car. Then here's what I'd do, (I had two black PT Cruiser's) wash only in the early hours or after the sun is out of the sky or in the shade, never wash in direct sunlight. Immediately dry the car with a 100% cotton towel. I always used my leaf blower to get the excess water off the car first. This alone will eliminate most of your water spotting and streaking. I also use a detailing spray after drying the car.
The problem is there are a lot of chemicals in tap water. The sunlight dries and hardens them on your finish if you don't dry the car off. I'd also suggest you get a good carwash soap, always use a clean bucket and make sure you regularly polish/wax your car. There are a ton of excellent products out there.
Mothers, Zaino, Meguiars.
Good luck and check out the detailing section, lots of good advise there.

Pat