You should use an anti-corrosive agent with the water in your radiator.
Most modern engine's use an aluminium cylinder head, on some engine's also an aluminium block.
Water is corrosive to aluminium.

Coolant in the right amount will not clog your cooling system..

Too much coolant will be bad for your engine.

Coolant doesn't actually improve the heat transfer capability of your engine.
What it does is allow the water to have a higher boiling point before it evaporates.
Only by remaining liquid will water/coolant/fluid in your radiator be an efficient heat transfer agent.