The car temperature tends to go up on a long distance and you have to hard more water?
high temperature and water not enough on 2000 Volkswagen Golf
Most likely you have a cooling system leak. Have your car's cooling system pressure checked ASAP before your damage the engine. I would drive as little as possible until you get it repaired and carry a couple of jugs of water with you when you do drive. VWs tend to get little coolant leaks. Good Luck!