First advise: never never add cold or even ambient temp coolant/water to an overheating engine; there is a very high risk of cracking the engine's head or block. Always turn it off, and let it cool itself down to say 60C minimum.
You need then to find where the coolant is leaking: could be external or internal. External leaks are easy to spot: you will see a puddle of coolant under the engine, maybe it is not dripping right down because the coolant may travel over parts, plastic covers, etc but at least you can see puddles of coolant somewhere. Look also for rusty trails, if there is a leak, it may be rusting stuff internally and/or externally. Look in the hoses, squeeze them while the engine is idling and moderately warm, look around the water pump, specially the shaft where the pulley is, also more complex to inspect, look around the heater hoses and radiator.
In fact, sounds silly but also look inside the car under the front floor carpets. Any sign of rust and humidity could point to a leak in the heating system. Pain in the neck to spot.
As for internal leaks, meaning the head gasket is gone, you may see transparent/white fumes in the exhaust, or a kind of mist there. But more common is: look the oil cap in the engine, coolant leak will blend with oil and make "mayonnaise", a kind of identical glop you can see inside the oil cap. Oil stick may not get this, but you will see oil like fogged.