Water

on Mars can processed with Moisture Vaporators or harvested from subsurface permafrost deposits via Water Extractors. It is distributed via a pipe network, which it shares with the distribution of Oxygen.

Apart from being essential for supporting life inside Domes and field buildings that have human operators, water is used for the synthesizing process of polymers and rocket fuel.

Background
The most important ingredient for life as we know it to exist is by no means scarce on Mars. However, it is mostly frozen and underground, making it difficult for acquiring. As bringing the water needed to sustain a city on Mars back from Earth will be a huge burden on every craft and far from efficient, constructing a steady water supply solely by harnessing water ice found on Mars is absolutely essential for the survival of any colony.