Usually the recommended PSI range will be written on the side of the tire itself as well. This can be hard to see depending on how dirty the tire is.

Don't use pure water if you live a cold climate, or it will freeze in the winter and blow out the bottom of the wiper fluid reservoir, or possibly do worse things.

If you lose enough coolant and the engine overheats to the point of failure, usually it will be a total loss on the engine.

However, I've never heard of anyone loosing coolant through the overflow cap.

Rotted hoses or hard pipes, sometimes the water pump (which actually pumps the antifreeze, not water), and a few other locations are the usual suspects for coolant loss.

If you have a blown head gasket, the coolant will mix with the oil and you can slowly lose coolant that way. This problem is usually very easy to diagnose, however.

You should only be adding pure water if the situation is absolutely dire. If you live in a colder climate and that water in your radiator freezes, your vehicle will be inoperable and you will be looking at a thousand dollars in repair, at least, if not a total loss.

People in warmer climates, where it never hits freezing, often think they can just add water; however, anti-freeze is still preferable, as water will potentially cause rust buildup inside your engine components.

It is very difficult to conceive of a situation in which someone would be unable to go to the store and buy antifreeze.

If you've purchased a used vehicle and you're unsure if it has the correct anti-freeze/water ratio, there are inexpensive tools (approx. $12) you can buy at the parts store which will tell you if its in the correct ratio or not.

Typically a 1:1 ratio, or 50/50, is recommended.

Finally, keep in mind that modern cars often use a specific anti-freeze blend. Almost all cars from before the 90s just use the green stuff.