Water is the source of life - and in Fallout Shelter, that's not an exaggeration. A stable supply of water, like food, ensures that your dwellers are happy and enjoy a full health bar. Without water, they're angry and more importantly, become irradiated. This reduces their maximum health, down to 50% of their basic hit points. Worse yet, it persists and has to be removed with RadAway or (slowly) disappear over time.
Water is a secondary priority. Power comes first.
There are three basic sources of water:
- Water treatment rooms, available from the start.
- Water purification rooms, which provide lots of water in the late game.
- Nuka-Cola bottlers, which provide lots of food and water in the late game.