Is there enough vitamin B12 in lakes, rivers and ponds, for humans not to need to take supplements?
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. 

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