Do you have a river or lake or other body of water connected to your city? Put some water towers near there and it will never run out. Or, if you don't, there's a way to get unlimited water by filtering sewage with either a water treatment plant or sewage treatment. Not entirely sure which one it is. Put them next to each other (with the filter pumps) and you will have an unlimited supply of water, unless your sims don't shit as much as they drink.