Pipes - These transmit water and carry away sewage. You need powered water pumps to generate water.
Water Pumps - Powered pumps will generate water for your city. The amount they produce is increased by placing them next to standing water. It also has a seasonal variance depending on rainfall.
Storage - Water tanks store water to combat seasonal variation.
Treatment Plant - Adding a treatment plant reduces your city-wide pollution levels.
Desalinization - Water pumps will not pump sea water. A Desalinization plant will take sea water and produce pure water for your city.