for maximum efficiency of the cooling system - ensure that there is no rust and mineral deposits in your engine and radiator galleries - if there is then clean them out - for maximum heat transfer to radiator you need the coolant/water to touch metal not rust and limescale.
Using a coolant also prevents cavitation which itself is very destructive (sonic cleaners use this very principle), cavitation can remove metal like it was cheese. I always suggest to use distilled water because it is the best solvent - it does not contain any salts that may ionize and then precipitate out at high temperature and cause deposits again.
First, get the system squeaky clean - then fill it up with coolant/distilled water mix.