Water is one of the most important Resources in the game. Water is necessary for making food and supplying Oxygen, but does not need to be directly consumed by Duplicants for survival (despite its description).
Fresh Water can be obtained by filtering Polluted Water using a Water Purifier, melting Snow/Ice or cooling Steam. It also can be generated by heating up Polluted Water to boiling temperature and then collecting and cooling down the steam (Distillation).
A Geyser produces periodically new water and steam.