Too much water?

Ok, the first thing I want to say is that I am not a medical expert, so please take that into account when reading this post. What you're describing is often called water intoxication (the big, fancy word for it is hyponatremia). It comes down to the amounts of sodium (salt) you have in your body.

if you drink too little water, your sodium levels rise and this leads to dehydration.

If you drink too much water, your sodium levels drop and this leads to water intoxication.

Both dehydration and water intoxication have the potential to be deadly (there have been cases of marathon runners to over-hydrated with fatal results). However, keep in mind that just like you have to be VERY dehydrated before you start experiencing problems, you also have to be VERY over-hydrated to have problems with water intoxication. Odds are your body would tell you to stop before you ran into any problems.

As sensitive as our bodies seem to be sometimes, they are actually remarkable devices that have a very keen knack of staying within homeostasis (read: keeping everything a-okay). God certainly knew what he was doing when he created us!