If sufficient rust inhibitors are in your coolant, regular tap water
should be fine, but that's a big should. Best bet is to spend that whopping couple bucks for distilled water and know that there are no minerals in your water.
SAE J814, 1998 SAE Handbook
Quote:
Clean potable water, low in chloride, sulfate, and hardness is generally acceptable. Brackish or undrinkable water should not be used. Softened, deionised, or distilled wateris ideal to dilute engine coolant concentrate. With properly inhibited coolant concentrate, water meeting the requirements of Table 1 is acceptable. Water impurities execeeding these levels can cause excessive scale, sludge deposits and increased corrosion.
Table 1 - Water Quality Property Requirement
Total Solids (maximum) 340 ppm
Total Hardness (maximum) 170 ppm
Chloride ( as NaCl ) (maximum) 40 ppm
Sulfate ( as Na2SO4 ) (maximum) 100 ppm
pH 5.5 to 9.0
Since I'm way too lazy to measure my tap water for these metrics, I buy distilled. 
