All the organs and cells in your body need water to be able to function properly therefore, you need to always replace water lost through breath, urine and sweat. If you are not drinking enough water, it can cause symptoms like muscle cramps, dizziness and fatigue. http://www.fitnesshealths.com/what-will-happen-when-you-drink-too-much-water/
However, drinking too much water can also lead to a serious condition known as water intoxication.
When you drink a lot of water, it increases the amount of water in the blood.
The water can dilute blood electrolytes, especially the sodium. When the sodium levels fall below 135mmol/L. http://www.fitnesshealths.com/what-will-happen-when-you-drink-too-much-water/

Smiles....
Too much of everything can kill na!