What effect does water have on a radiator that is already using coolant.?

I believe water causes metal to rust. Even if its an aluminium block your radiator is made from iron some times and some others copper.The boiling point of Clean water(H20) is100 degrees Celsius or212f, With all the salts like Magnesium, calcium and chlorine(in tap water) you can have it boil at less than85 celcius. These salts deposit after after long use of water( same whitish stuff you seee at the bottom of kettles)and the deposits clog your cooling lines and stresses the water pump.As water boils it expands and also releases gases dissolved in it.Gas expansion due to heat is way more than liquid expansion. Your car cooling system is a close system and this expansion causes pressure build up. At some point some thing has to give way, either the water pump, hose, or radiator.Coolant is made up of mainly TEG ( Tri ethylene glycol) and silicates. TEG has a boiling point above320f and low expansion due to heat. New cars have a shuf off system that would tun off the engine when temp rises above a certain point.Pls do not use water if you want your engine to last long

It depends on the situation you have

Regarding the salts in water

If the water is% pure (demineralized water ) there will be no effect 

If the water is fresh (has some salts) the corrosion will occurs after long time 

If the water is salty (has high salts concentration) the corrosion will occurs in short time

  Regarding the temperature of water

As temperature increases in case of low efficiency of colant the corrosion rate will incueses