Storing water is difficult because it is both bulky and heavy in quantity. Nevertheless, if water supplies were interrupted then simple everyday tasks like cooking, washing and using the toilet would suddenly become considerably more difficult.
The World Health Organisation suggests that around three litres of water per person per day is the minimum requirement for survival. Storing a few days’ worth of water, whether pre-prepared bottled water or filling suitable sealable containers and keeping them somewhere cool and dark, is certainly a good idea if it’s possible for you to do so. This is especially true if a substantial proportion of your food stores involve dried, rather than tinned or long-life food; anything that will need to be soaked or reconstituted will require some water as part of its preparation.