Water (Fallout 4)
Water is absolutely essential for any settlement to function; no water means no settlement. Fortunately, water is one of the easiest resources to acquire, only requiring the building of a pump; popping a pump down into open ground will immediately begin generating water for the settlement. However, single pumps produce small quantities of water; large scale water manufacturing is possible, but the purifiers (both normal and industrial grade) require lots of materials and an open body of water in which to operate.
Water pumps, both large and small, are susceptible to damage during raids, and may need to be repaired in the workshop periodically.