Clean water is essential to life. One of the reasons that cities in the Middle Ages could only grow to a certain size was the lack of pure, since, regrettably, much of the skill in building aqueducts had died with the Romans. Crafts such as tanning and the retting of flax require large amounts of water, which becomes polluted during the process. Such crafts were normally forbidden inside the city walls, and restricted to locaions downstream of the city.
In some times and places of the medieval era, water was the beverage of choice (or necessity) for the majority of people. In other times and places, mostly those with larger, more crowded cities, the drinking of plain water was considered undesirable and the drink of choice would be small beer, or for those with more money, wine. For example, Tudor era English ships carried barrels of weak beer rather than fresh water, which was probably safer, since alcohol slows the growth of bacteria.