Water can be used in a cooling system; as your cooling system is generally a 50/50 mix of distilled water and ant-freeze. However this should only be done in an extenuating circumstance AND NEVER in a cold climate location. The reason I say this is first off water freezes at a temp of 32 degrees Fahrenheit, so that means if its snowing out you run the risk of the water freezing inside your engine and causing major damage, such as cracked block, warped cylinder head, or cracks in the radiator or heater core. The vehicles cooling system is an integral part of the engine and without it running properly there can be major issues; it also travels throughout the engine including the intake manifold. As Coolant is also Antifreeze which means that its freezing point is MUCH lower than regular water.
Even in hot climates where water will not freeze and cause major damage it can still cause issues. Like the name suggests, coolant, is responsible for keeping the engine cool in relative terms, imagine the thousands of mini-explosions occurring in your engine as its running, these are what generates the energy to propel the vehicle, and basic physics tells us that energy is heat (more or less). This heat can build up quite rapidly, to the point where it can blow out a head gasket, warp the cylinder head or worse! Such as burning the piston rings and causing the engine to seize, melting holes into the top of the piston or binding a rod bearing and blowing a hole through the block! Most if not all cooling systems today are a pressurized system which in itself increases the boiling point of water, but like how Coolant/Anti-freeze lowers the freezing point of the cooling system, it also increases the boiling point further.
In addition to causing damage as stated above regular water has minerals in it and those tend to build up inside the motor, preventing proper flow of the coolant (this is why radiator sealing products can cause more trouble than their worth, but that’s for another article)
If you need to use water in a pinch try to use distilled water as it is free of impurities.
To further touch on the subject, more and more manufacturers today are using aluminum alloys and new compounds in their engine assembly and gasket materials, and they formulate a coolant specific to their needs. It may not seem like a big deal as there is universal coolant that mixes with everything. However there are long term effects. True the products DO in fact mix with any brand or type of coolant and they work, however they too can have an adverse effect. Due to the particular composition of the materials used each manufacturer has formulated the coolant that will suit their system best, Teams of researchers have toiled and developed an ideal liquid that does not strip away any of the elements in their alloys, or work best in their pressurized system. Every major automobile manufacturer has their own brand of fluids and recommends that you use them, because they are formulated for their vehicles. It is wise to abide by that. There are some solutions offered if you care to change the cooling system to another type of fluid such as Water Wetter (for use in racing applications) is designed to bond well with a primarily all water solution. There is also waterless such as Evan’s Waterless Coolant which is arguably the best solution as water has a tendency to corrode internal parts especially if it’s a vehicle that sits more than it drives, and they also do not break down over time so essentially you could have a truly sealed system.
Ultimately it is generally not recommended to run with an only water solution or a highly water biased solution in the cooling system.