For the squeamish, don't forget that our colons and digestive tracts are normally teeming with billions and billions of bacteria anyway so it's not like we're fouling some sterile environment in our bodies. And the lining of the colon is essentially the same thing as the semi-permeable membrane from a reverse osmosis filter. So, whatever nasty water you introduce using an enema, in all likelihood, whatever shouldn't get into your body will just get left behind in the colon and then expelled in short order.

Those who suffer from constipation see similar effects--moist digested matter moves slowly through the colon, the colon continues to extract more and more moisture, until the final form is rather dry and hard.

Of course, introducing the wrong little bugs into our digestive tracts, could lead to a sort of "food poisoning" or infection but then again, maybe not.

I'd say that it's a physiologically logical method at extracting desperately needed water from certain sources, although not necessarily the most appealing idea. And there some risks, too, like from a contaminant in the water and the risk of injury from performing an enema out in the field.