While it's true that Humphrey Bogart and John Huston avoided nasty diseases by drinking whiskey instead of water while filming the African Queen, the idea that beer/ale/wine replaced water is often badhistory as the wiki points out
https://www.reddit.com/r/AskHistorians/comments/2h379z/i_once_heard_it_said_that_beer_is_partially/
/u/Qweniden gave good answers.
I read that Beer/Cider/Wine was drunk as water to poor in quality to be drunk in middle ages.
This is frequently found in books (especially general readership books) but this can not be substantiated with primary sources and fails many logic tests.
The main point to consider is that poor water quality is a problem when you have high population density in an area without an adequate sewer system and no infrastructure to bring in fresh water. For starters, in the middle ages there were very few high population density areas in western Europe. Until the 18th century it was a world that was by far almost exclusively rural and agrarian. People built small villages and homesteads in areas where they had easy access to fresh clean water and eventually this was even easier with the advent of artesian wells. Lack of clean water was just not an issue faced by people prior to widespread urbanization.
But even in the biggest cities that did exist, you might be surprised at how sophisticated water delivery systems were in the middle ages. London had the "great conduit" for example. It seems that the poorest of the poor would have had to drink river water but they were a minority in a minority. Clean water was simply not an issue for the average person living in the middle ages.
But by the 19th century water was clearly a large part of the diet again (as seen from the amount of people infected by cholera in water pumps). When did this shift back to beer to water occur?
There was never a shift. 19th century London had the cholera outbreak because population density was finally outstripping their ability to have clean water.
Wine during the middle ages was seldom drank pure. It was usually diluted (because of how strong and expensive medieval wine was) and/or flavoured and sweetened, defeating the purpose of not drinking possibly tainted water. In areas which were occupied by the Roman Empire, you'd usually have a really elaborate system of aqueducts to bring fresh and clean water. Polluted water wasn't really an issue since most people lived in rural areas.
edit : Source for the diluted wine : The Wine Trade in Medieval Europe 1000-1500 By Susan Rose (p138)
There's always more room for discussion, but you may be interested in the "drinking water" section of our wiki, which points out that your basic assumption (that people drank alcoholic beverages to avoid contaminated water) is incorrect:
https://www.reddit.com/r/AskHistorians/wiki/health#wiki_drinking_water
Does anyone have any information on such drinking habits in South East Asia in empires like those of Siam and the Khmer? With regards to /u/GrandStaircase 's assertion that these issues were prevalent in high population density urban environments, knowing that Angkor [as an example] was a massive pre-industrial settlement, did these issues crop up and if so, do we know how they were solved?
Are there any experts in Polynesian practices here? I'm aware of a system of spiritual restrictions (tapu) which limited water sources to certain uses depending on its cleanliness but am not qualified to really expand on it.
In Russia and Eastern Europe there is a drink called Kvas made from fermented bread. It has an extremely low alcohol content but that was enough to kill off must of the yuck in there. It was first mentioned in the primary chronicle in 996AD. It is still in production today and if I had to describe the taste it is like flat coca cola.
