Vegans will often say that having an animal-free diet is best for our health and is actually the diet that humans should be eating to function optimally. To which meat eaters will raise the issue of B12, and how (they believe) it is only to be found in animal products [thus vouching for an evolutionary need to eat meat and eggs). These days, with all our water being treated to remove pathogens, it is obviously best to take b12 supplements, but the question remains: do vegans have a point? Is there actually enough b12 in water for humans to not develop an insufficiency?
Of course there is no evidence for the superiority of the vegan diet claim to begin with, so there is that, and there are plenty of vegan foods fortified with B12, even some from non-animal sources.
No, there is not enough B12 in treated water nor well water for a human. There can be enough B12 in untreated river/lake water because it contains high levels of fecal matter or algae which is high in B12 (b12 is manufactured by anaerobes). Of course these are the things you want removed from your drinking water so reducing B12 is inevitable. Also can contain enough being the key phrase it depends on the water source, cleaner river contain less.
Evolutionarily speaking humans eat animals for the same reason every omnivore and many herbivores (deer, ect) do, because we can, it is generally a superior nutritional substrate which is more than enough for evolution to favor it.
Fermented food is a safer and higher b12 option for a vegan, it can have a lot higher content of yeasts and bacteria than safe water, and it can be controlled.
Most vegan foods (not protein rich) can safely ferment until 50% to 99%(sry for the humor) of their expiration date, living plant matter in any carb/plant mix will generally kick off a good fermentation. some foods like salad couscous are full of yeast and become unappetizing and gluey with yeast a long time before they become unhealthy to eat.
It would be difficult to design a pond/lake which has high content of bacteria and archea and chlorella to be optimal for humans, whilst also being safe for children. Rivers should have mats of algea that filter the water, however they are so variable, it's pot luck if a river has an excess of nitrogen or a silty bacterial nuisance, A river can be jam packed with nitrogen detritus, and after a storm/ in the wrong place it's not reliable.
In winter the lake can become less productive biologically, too high in nitrogen, and if it's biologically rich it will be subject to wild seasonal variability.
Blooms of animals, daphnia, cyclops can occur, eating whatever is making the vitamin b12, and any unseen bacteria can bloom, which benefit from warmth and that will thrive noxiously in the gut.
A snail doesn't even have a lot more neurons than a tardigrade, and definitely a lot less neurons than small insects and acarians that you accidentally consume from plants.
You can reprimand me for the lack of references, it will give be a subject of study, however it's too broad a topic to reference and research right now, unless you think i've made an overstatement.