This car is losing radiator fluid. Don't know the cause. Wondering if the fluid is low, if we can fill we H2O or should we use antifreeze?

Water ok .. 'until fixed' .. >ASAP!

You should get it pressure tested for leaks. I also had the same problem with another customer where the water pump only leaked once in a while. Also make sure you bleed the cooling system that can cause it to overheat 2. How to bleed your car vid. https://youtu.be/ZEnExhpW8pk