You are taking about the ECT (Engine Coolant temperature) gauge. Ideally and in OEM setup it should stay at center (50%) +- 10%.
Oem recommends ethylene glycol based coolant in 1:1 (50/50) with distilled water.
- It will protect the complete cooling system from rust and corrosion of head and engine block aluminum alloy.
- It is very effective anti freeze and protect against freezing of the coolant till -37C
- It will reduce steam in the system by increasing boiling point to 107.2C
- It is also said to have lubricating properties helping the water pump and thermostat valve movement.
And as far as my knowledge is concerned the thermal conductivity of ethylene glycol based solution (1:1) aka coolant is less than that of plain water So if it is just all about thermal conductivity then plain water has no competitor BUT there is lot more in the cooling system than just heat transfer.
To rectify the issue and to avoid re-occurence.
Use good quality coolant in 50/50 percentage with distilled water, not just green water.
Ensure that radiator fan is working optimally , the shroud is properly fixed and is not sucking air from sides of the radiator instead of through it.
The thermostat valve is there to regulate the coolant flow which ensures proper contact for heat absorption from the engine head and cylinder walls.
The radiator should not be blocked internally or externally.
Check and replace if necessary the metal pipe going to the water pump, if using plain water as coolant you will find it chocked.