Most water spots are from hard water. A water softener will help significantly. A clay bar followed by a good wax/polish will help too. Don't skimp on these, go higher end like mothers or mequiers. These are leaps and bounds ahead of those spray on walk away waxes.
Once its clean there are a few steps you can take to keep it clean for more than an hour. Park in a garage, a portable garage, or put a cover on it while not in use. A bit more maintenance than most other vehicles, but thats the cost of black.