Water spots run the gamut from super easy to remove to almost impossible, but you don't know what you've got until you start working on them. Sometimes something as simple as a cup of distilled white vinegar mixed with a gallon of water will do the trick, sometimes you need a rotary buffer and a heavy cutting compound. Those are the extremes and I'd definitely give the vinegar/water wash a shot first. If that doesn't do the trick then pick up some Meguiar's ScratchX 2.0 or Meguiar's SwirlX and give that a pretty aggressive application in small areas at a time - no larger than roughly 1 foot square. Wipe off BEFORE the product dries. It's best to use a soft foam wax applicator to apply these rather than a terry cloth towel because terry cloth has some bite to it and can haze the paint when used with a lot of elbow grease.

Regardless which process you choose, make sure to wash and dry the vehicle first.

As for prevention..... that's the tricky part. Obviously you should never let your car air dry after washing it. Keep it away from sprinklers. Far away. Sprinklers usually use untreated (ie non softened) water and as such they often contain minerals and other things that can actually etch into the paint if allowed to sit and bake in. And anything that can actually etch the paint is not going to be stopped by any wax on the market today. Which is why the worst case involves rotary buffing to pull the spots out.