What causes the car to overheat even when water is added

I would check and replace t stat for starters

You may have a large air bubble in your cooling system. Put the car at an angle with the front raised higher than the rear (a driveway is good). Remove the radiator cap, start the car and turn on the heater. Rev the engine to about 2500rpm for a minute and watch to see if the float goes down. If it does then keep reving for another minute. Slowly add coolant until the float rises even with the spout lip. Repeat the procedure until the float stays in one place. The other issue may be the thermostat.