On the subject of medieval (and antiquity) "spiked" bewerages, last summer I read The Immortality Key: The Secret History of the Religion with No Name by Brian C. Muraresku. It would make a starting idea for a wonderful ars magica saga. You can hear the author's interview on youtube https://www.youtube.com/watch?v=Jdf_CJ7stwI&ab_channel=SamHarris
Also, I don't know how widespread it was in wider Europe, but after doing some research for my own sagas, I have the impression that at least in medieval France, there was a religious community every few kilometers, and some of the most important owned a huge portion of the territory. As an example, it is my understanding that the abbey of Remiremont owned something like the quarter of all Lorraine in 1220...
I do not understand this. Out of built-up areas, I would drink untreated surface water any time, well, running water, that is. Why wouldn't I? And why would this be worse in the middle ages? I can see the point in areas with dense populations or little throughflow of the water, but does that account for the majority of Mythic Europe?
My understanding is that this is something of a popular myth or an outdated view.
Whilst it's possible that the nobility had most of their fluids mixed with weak wine and this could have been the origin of the myth, outside of cities, people did indeed drink water most of the time.
... that's a bit of the problem with germs and parasites. The more you avoid them, the more vulnerable you become ...
We did camp with a foreigner, once, who was not used to drinking from a stream, and he returned the next day with a stomach bug, blaming the water. Nobody else was remotely affected.
... but whatever the middle-agers drank, I am sure they were used to it ...
To a limited degree. Constant low-level exposure can keep a healthy person's immune sensitised to a pathogen so that you need a higher level dose of the pathogen to overcome your thresh-hold of protection (I am not a medical expert, just trying to pay attention). But germs mutate fast allowing for regional variations so that you can drink your local clean river water safely, but not the river water a 100 miles away that the locals can stand - I think it is often called Montezuma's Revenge, or Bali Belly.
Doesn't work so well with tetanus, plague, and numerous other diseases both endemic and fleeting from the Medieval period, especially concentrated in urban areas. Of course, in ME there are more witches providing magical medical aid.
The above requires a healthy immune system to begin with, something the average peasant may be lacking from time to time with a less-than-superb diet, exhausting labour, no refrigeration to keep food fresh, no modern antibiotics nor vaccines.
In modern, healthy times we can more or less safely ignore the old wives' tales that you will catch a cold if caught in the rain. But those old wives tales are based off of observed conditions centuries ago.
There is an enormous variation in local factors, but the rule of thumb I use is that 1 in 3 children will not survive to adult-hood due to diseases, parasites and other pathogens.
We've had nearly a century of film trying to paint the middle ages as dirty and squalid.
Whilst it is true that modern people are very much more aware of hygiene than our medieval ancestors, medieval people were to an extent often cleaner than people during the Industrial Revolution due to the lower population density.
Assuming these would be Folk Witches, I'm not sure they're providing much aid -- their Healing requires vis, so they won't be saving the entire village from the trots.
In my campaign I swapped Mythic Herbalism for Healing with their local witch, to tie in with the covenant's Pharmacopian: that would be effective magical aid against bad water (or bad airs).
I hadn't factored in hygiene in my analysis. I am not certain how difficult hygiene is without easy running water, just the cold water lugged by bucket from the local well or stream.
In the countryside people lived much closer to somewhat unhygienic animals and livestock.
While in walled urban areas sanitation was more overloaded and sewerage often not so easily disposed of. I can't imagine what it was like near the Shambles or Tannery.
Exactly how dirty and squalid is difficult to determine, possibly varies depending on sieges and famines.
What water you could drink would really, really depend on your location. People still dug wells, and well water was probably safe - especially for the locals, who would be used to whatever was in the water. Drinking from another village's well, you were taking some chances.
OTOH, if there was a spring or artesian well, and the water was considered 'sweet' (i.e. not poisoned with harmful minerals), almost anyone could come by and have a safe drink from it. Springs created by magic would fall into this category.
River or pond water was just asking for trouble, you wouldn't drink from that unless you knew it was safe (like a small stream that was fed by a nearby spring).
Magical and faerie influences will make all of the above highly variable, of course. Maybe a faerie would keep a well's water safe in return for attention, or a river spirit might fastidiously keep his waters 'pure', or an infernal spirit might poison waters to keep people miserable.
Something to keep in mind is that most of recorded history was recorded by people who were literate, which meant well to do. People with multiple estates would travel between them, and thus not build up a tolerance to the local pathogens. as such the people recording history in the middle ages probably drank much less water and more spirits (which they could afford) than average.
That being said much beer in the middle ages would be closer to a protein shake of today than anything we would think of as being purely a beverage. Drink it down fairly quickly in the middle of the day and back to work. Water had a reputation amongst the nobility as being unhealthy to the point that some even bathed in wine, but the peasants drinking it wouldn't mean much because after all everybody 'knew" that peasants were filthy and disease ridden anyways.
As with a lot of things in the middle ages there is not just one answer but many perspectives depending on who and where you are.
Indeed, although I think we have drifted off topic.
The subject of "People drank so much ale/weak wine it replaced water" is untrue, so lets go back to what things people have found surprising that actually did happen,
Winter travel was basically unheard of. The few winters worth of reports I've seen or read about were snows so deep that all trade and travel straight up stopped for, often times, months on end.
Once deep winter set it, people simply tried not dying until spring.
I think this is an over generalisation.
Certainly the types of travel undertaken would be different and this would vary by country and latitude, but to say that it was unheard of would be a vast oversimplification.
Armies generally didn't move during winter because they were dependent on horses and horses consume a lot of fodder that is not as available in winter.
However people did move around in Winter, particularly within the Medieval Warm Period.
In France for example the fair of Lagny-sur-Marne began on the 2nd January. Which would have necessitated merchants and customers travelling to the site of the fair.
There are mentions of Winter Pilgrims even in England.
As @loke said, nobody writes about a calm and regular winter.
But the trying not to die part is fairy interesting, in places with really cold and long winters where nothing could really grow, the only things needing to be done were to tend the animals and cook food, so the rest of the workforce on a farm essentially huddled together for warmth in their beds and entered a human analogue of hibernation.
As for my contribution to the actual thread, despite a lot being written mentioning the use of coinage, it was mostly restricted to the nobility and landowners. While a peasant would essentially get a wage, it was mostly paid in trade goods.
It has been said that without ham and cheese, much of northern Europe could have been settled. Ham and cheese is some of the very few foods that preserve across winter and don't need a fire to prepare for eating.
So don't expect a lot of fresh stuff in Winter meals