--one absolutely needs water, as that drug is not metabolized by the liver, but excreted (almost) whole by the kidneys, and one need the water to properly eliminate it.

I think the danger is from flushing out too much sodium and other minerals from drinking large amounts of fluids if one is also on diuretic meds. This, again, would be more likely to happen with those on restricted sodium diets (since the standard American/Western diet has more than enough sodium in it to counteract all but the most extreme diuretic regimen).

I would imagine anyone on such meds who was also on a low-sodium diet might want to mention this to his/her doctor, so either sodium intake or the meds themselves could be tweaked.