I just read that the average wine consumption for medieval person was 3L/day as it was used massively as a drinking, cooking, healing ingredient and also that the water was too "polluted" to drink. How polluted? (Might be biased cause I'm French and study was about medieval times in France )
More can be written about this, but the myth of unclean drinking water has been addressed in discussions here before. I think the best discussions are the replies of /u/Qweniden in "What factors made beer so important to the establishment of civilisation? What made it a more practical drink than plane water?" . It's pointed to from the FAQ area "Health / Hygiene And Healthy Lifestyles" .
This is not to discourage discussion. Further questions, data, and debate are welcome.