Never use straight water in the cooling system. Never. In your cooling system, coolant provides three very important benefits over water. It has chemical additives that (1) prevent engine corrosion and it not only (2) raises the boiling point but also (3) decreases the freezing point.
Concerning the OP's problem, when water freezes to form ice, it expands with a force capable of breaking rock and bending metal. Therefore, the main concern is possible damage that could occur if ice formed inside the radiator or engine.