Aside from the fact that adding cold water to a hot engine isn't a good idea, why is the coolant low?
If the system is full, you shouldn't be able to add anything.
If the engine is hot, you shouldn't be able to take off the "rad" cap, at least not without a scalding shower and serious burns.
If you are regularly topping up the coolant then you have a leak somewhere that you need to find and correct. (its supposed to be a sealed system)
An engine thats running too hot will indeed cause "heat soak", but for all the wrong reasons.