Well, water is actually better than coolant as a cooling agent and one can actually run car on water without any problem.
However, coolant has higher boiling point (110+) than water (90-100) so when car is little low on water it heats up faster and water reduce faster due to less boiling point and cause all the trouble by over heating the car. Second reason to prefer coolant is its anti rust property that makes radiator rust free and doesn't clog the radiator cores after sometime, if you drive car with water only then in six months time car radiator will be blocked due to rust.
As a mixing with coolant, or just topping up in case of emergency, or using full water in case of emergency is acceptable but you need to flush whole water and top up with coolant as soon as possible in next few days or within week time.