Hard water often contains large amounts of magnesium and calcium. Not sure how that would affect kitties or humans for that matter. All I know is that many experts recommend giving pets filtered or spring water. Some even suggest using distilled water.
I've always read that distilled water is a no no with cats. Too many minerals are actually removed from the water that kitties need. From littlebigcat.com:
Distilled water has been purified so that it does not contain any particles at all. While purity may sound good, you really should not use distilled water for drinking. The reason lies in the fundamental nature of water. Water and solutes (molecules and particles) move by osmosis and diffusion, respectively. You might remember these from high school chemistry class! Basically, water moves by osmosis from where there is more of it, to where there is less of it; and solutes diffuse from an area of high concentration to an area of low concentration. Distilled water contains zero solutes, so when it enters the intestines, diffusion will actually pull solutes out of the body. Drinking only distilled water can ultimately cause deficiencies in sodium, potassium, and important trace minerals. It also becomes acidic when exposed to air. In people, exclusive consumption of distilled water is associated with high blood pressure and heart arrhythmias. While distilled water can be valuable when used for a short-term process of detoxification, it's not safe for long-term consumption.
Full article here:http://www.littlebigcat.com/index.php?action=nlarchive&show=volume3no061