Technically speaking yes you can use plain water in your cooling system but it isn’t recommended as a long term solution and certainly not in extreme weather conditions.
The problem with using water in your cooling system is that water freezes at 0°C. In wintery conditions this is going see any untreated water that you have put into your cooling system begin to freeze over causing serious damage such as cracks and warps in your engine.
Even if it is the height of summer, failing to put antifreeze into your coolant system can be detrimental to the health of your vehicle. The operating temperatures of engines differ from model to model but it is not unusual for the operating temperature of an engine to reach above 100°C which just so happens to be the boiling point of the water inside your cooling system.
Boiling water in your coolant system will cause you a whole host of problems. The main function of coolant is to direct heat away from the engine which is a by-product of the combustion process. It is natural for the coolant to heat up during this process but if the water is boiling then it isn’t going to be doing anything to cool your engine down. Additionally, if tap water has been used in your cooling system and is allowed to boil and evaporate then then this can increase the chance of build-up and corrosion in the engine because of its naturally high mineral content.
Using antifreeze in your cooling system, diluted to the recommended specifications as outlined in your vehicle’s user manual, serves a number of functions. It both raises the boiling point and lowers the freezing point of the water in your cooling system which means that it is much more effective at regulating the temperature of your engine more efficiently than water alone. Using diluted antifreeze of the type recommended for your engine also carries the additional benefit of additives which protect your engine from corrosion and build-up.
If you find yourself without a choice and having to use plain water in your cooling system then make sure where you can that you use distilled water as this has a lower mineral content than normal tap water and has less chance of causing build-up in your engine, and top up the system with antifreeze as soon as possible.