Go to an auto parts store or Wal-Mart and pick up a clay bar kit. It should include a bar of clay (literal clay), a lubricant solution, and a microfiber towel. Wash the car then clay bar it. It will take a few hours, but follow the directions on the box and clay bar the entire car. The clay bar process will strip the old wax off the car so you'll need to wax it again. Expect the whole ordeal to take an entire afternoon but your car will look fantastic when it's done and the water spots will be gone. I had a white car that was slowly getting covered in small brown spots that looked like rust (was actually surface stains on the paint) and after clay barring the car, all the spots were gone.