Before the 19th century, drinking water had the potential to make one sick because of poor sanitation. Practical experience showed that fermented beverages were less likely to produce illness. At mealtimes in the Middle Ages, everyone could drink small beer, including children, while eating a meal at the table. Table beer was around this time typically less than 1% ABV.
It was common for workers (including sailors) who engaged in heavy physical labor to drink more than 10 imperial pints (5.7 litres) of small beer during a workday to quench their thirst. Small beer was also drunk for its nutrition content; it might even have bits of wheat or bread suspended in it.
Source: Small beer – Wikipedia