Comment by BobaFloutist

It would be a really cool coincidence if the temperature at which water boils happened to be the magic number that killed most bacteria (unless it killed them because it boiled water they depend on to function or something), but I don't think that's the case - I think that boiling is something that's easy for humans to do and to perceive that also happens to be above the safe threshold and kill enough of the bacteria that harm us to make it a useful break point, even if the actual 212* isn't especially relevant.