The basic amount of water to store for drinking and food preparation is usually reckoned to be 4 litres or 2 large pop bottles per person per day. This goes up to 8 litres a day if you want to use water for personal hygiene and 20 litres a day if you want to do the laundry as well. You can buy in bottled water but you could also rinse out old pop bottles and fill them with water. If you do this, mark the pop bottles with a date so that you can work out when the water has been in those bottles for six months so that the water can be changed. You will also need to check the use by date on the bottled water and rotate it with newer supplies.
Water can get a bit boring. Why not consider keeping some other items like cartons of fruit juice and bottles of blackcurrant juice. Some people will not be able to manage for very long without their tea and coffee and presumably dried milk to make it more palatable.
You should also consider how you would transport water from a standpipe if you had to depend on them. This might include having a trolley and some water containers that you would use when caravanning.
I suppose some people would assume that the heading drink refers to alcoholic drink. For some, this would seem a good idea as it would blot out the problems but that is a temporary fix as their problems do not go away and may possibly increase as a result. One possible problem of alcohol is that the body needs more water to be able to process it so increasing the demand for water which may be in short supply. The old saying “drink in wits out” is also relevant here as you need to keep your wits about you during an emergency situation.