I know some people with IBS are really sensitive to different tap waters (which presumably is what the guy who started this thread was talking about). And that's reasonable as they do differ and do have "stuff" in there. I'd say that if he'd switched to mineral water, he'd have been fine.
Not enough water will make C worse and can cause D. Don't go there. It is very important for some people (like me) to keep their electrolyte intake up as well tho. But that's just drinking water PLUS not less water.