Err, no, the devs can't handwave "it works!". The technology has to make sense in the context of the Fallout world and at least have the bare minimum presented for the end user to understand how it works.
For instance, FEV-II. It isn't a magical device to excuse all mutations. It has a defined way of operating, its effect on living organisms are elaborated upon in Fallout 1 and it is grounded in both lore and story. Or plasma weapons, their design is explained in the games in scattered clues.
Project Purity doesn't make sense from any perspective. You don't seem to grasp the difference between "a little bit different" and "wholly different", do you?
It turns one element into two others, the original is destroyed. I might've phrased it wrong, but you get the idea. Nuclear fission in nuclear bombs is all about releasing energy, so it's not a far fetched assumption that with the perfected nuclear technology of the Fallout universe, they might have devised a way to maximize yield whilst minimizing fallout.
As for access to potable water... There are a lot of contributing factors. Civil unrest, lack of access to water in any form, biological contamination of the ground, simply not knowing that it's possible etc. However, you cannot really compare these situations, as survivors of a nuclear war would certainly be much more desperate for knowledge and information on how to purfy water is contained generally in every good survival handbook. Just raid the library.