Yeah, never use tap water, in Arizona we have VERY hard water and the calicium deposites tend to screw up the water pump.

Don't use purified mineral water like aquafina or anything like that though, they tend to add stuff to them, just use plain old distilled drinking water.
 I have a 1970 240Z (restored) and a few years ago distilled water was used in the radiator instead of
 tap water.
 The results have proven to be unequivocally positive. The distilled water keeps the radiator
 absolutely pristine and there is no corrosion around any of the heater connections.

 Corrosion was a chronic problem until the switch to distilled water was made.

 I run about 5% to 10% coolant (this is Southern California) and I cannot remember the last time
 I changed the water pump. In the past I was blowing through water pumps about every 6
 months or less.

 Give it a try. I'm doing this to all my vehicles now. Use the distilled water in the
 radiator and window washer cleaner.

 Has anyone else tried this?
   
 

It's a very good idea. In many areas the water is awful and can be very
abusive to the cooling system.