Water can be found from expeditions in the wasteland and can also automatically be obtained at the shelter when it rains. Water is stored in water butts.
Water is also collected during black rain events, however the rain collected will gradually increase the dirtiness level of the shelter's water and will take time for the water filter to decontaminate. Survivors who drink contaminated water risk suffering from radiation poisoning.
While shower and toilet usage also requires water, there is no risk in using either of these when the shelter's water is contaminated.