Bad water spots can actually etch the paint, in which case they'll need to be polished out. If you are not able to remove the spots using a clay bar, distilled water/vinegar, or the CG waterspot remover it's a good chance your car needs to be polished to remove the waterspot etching.
Spend some time on Autopia.org reading the how to forums on polishing paint or find a detailer near your area by posting on Autopia. It's really not that hard, and once you learn how to polish paint its a nice way to make some extra cash on the side.
I spent lots of time on Autopia reading the how to articles, and finally bought a PC7424 and a Chicago Electric rotary along with an array of LC foam polishing pads and a few wool pads. The particular package I bought came with the PC, several foam polishing pads, a few bottles of polishing compound (fine swirl remover, moderate swirl remover, and a heavy swirl remover), and a glaze.
I'd recommend practicing your polishing skills on your DD if you have one. I polished my Grand Prix before polishing my Mustang. Learned a lot about detailing just by doing it.
Before shot of roof on my Pontiac (first car I ever polished):
Here's a couple 50/50 shots of the roof:
After shot:
When I detailed my Mustang, I didn't take any before shots but here are the afters:
Here's a couple good reflection shots:
The following pic was actually taken about 3 months after I polished the paint, and I had just polished the headlights.