Running straight water through a vehicles radiator is okay as a short term solution and in an emergency.
If you have a radiator leak and need to run water through the radiator to keep your vehicles engine cooled off and allow you to get home then it's okay.
But if you run straight water through your engines radiator too long it can cause rust inside the engine and corrosion inside the water pump and radiator.
So while you can run straight water through the radiator and engine however it's not recommended to do so for too long.
You should use coolant and antifreeze in the radiator to help cool the engine off much better and keep the engine from rusting and corroding.