Ask a doctor, but from what I've heard one has to drink extra water when having a diahrrea, to recover the lost liquids (ok, not the lost liquids, just an equivalent amount). I'm not sure what the consequences of dehydration are. Headache, blood pressure problems, it could eventually be serious. The alternative is serum direct to the vein but that's normally only possible in a hospital. Too much water can be dangerous but it has to be really a lot, like in normally one is not able to drink so much in his right senses unless forced. At least in "acute overwatering", so to say. "Chronic overwatering" is more likely, that would be having the habit to drink too much water (I don't know, maybe double or triple as a normal person) every day for a long time. That might overload the kidney, I have heard. But I have no idea about health.