Loss of micro nutrients
Water helps to detox your body by removing toxins that are built up from the air we breathe, the food we eat and other toxins that are built up in the body (like lactic acid from muscle fatigue). Water is also used by the body to regulate micro nutrients (such as water soluble vitamins and minerals). Sodium, potassium, and magnesium; for example; are lost through sweat and urination. Every time you urinate, you are losing some sodium. Most people living western lifestyles and eating typical western diets have sodium levels that are too high because they consume too much sodium (from salt) and drink too little water, but drinking too much water can drop sodium levels too low (especially if you have naturally low blood pressure or already follow a diet low in sodium). Sodium is needed in the body for many functions like muscle contraction, blood volume and blood pressure regulation. Some people advise that you should not drink so much water that your urine is completely clear, but aim for a balance where the urine is slightly colored. This, according to some, ensures for a balance where the body is hydrated yet not losing too many essential nutrients.
Too much water and your kidneys
Your kidneys are responsible for filtering water levels in your body. Drinking too much water all at once can put strain on the kidneys. This is usually not of much concern or something that most people should worry about, but keep this in mind when you try to get all of your entire 'daily water requirement' in at one shot. People with kidney malfunction are often given limited fluids to avoid extra strain on the kidneys. An average adult's healthy kidney system can handle up to around 15 liters of water a day (more than almost anybody would ever drink), so the effect of over hydration over a longer period of time on the kidneys is generally not of great concern.
Too much water and the brain
When lots of water enters the blood from drinking too much water, cells in other organs and muscles can stretch to accommodate this. This is not possible with the brain, because cells are more tightly packed. Over hydration can hurt the brain because of this. Headaches from over hydration can occur when too much water is drunk at once, but headaches from under hydration are more common within the western community.
The thirst argument
We have all heard that thirst is an indication that your body has already entered into a dehydrated state. Some doctors, researches and sports specialists argue that thirst is not as bad as it is often made out to be and that it is instead a good indicator, or regulator, of when to drink more water and when not to. Who is right? Decide for yourself.
This does not mean that you should not drink water!
Water is vital to human survival. Drinking too little water is more common than drinking too much. It is also more dangerous to consume less water than necessary than too much. Do not avoid drinking water when you are thirsty, in hot or humid conditions, exercising with a high intensity, sweating a lot or feel like you need it. Water is not only released from urination and the skin when we sweat, but also from the lungs as we breathe as well (see that water vapor when we breathe in cold surroundings?)
So there you have it: too much of anything can turn into a bad thing, including plain old water. Never push your body past what it can handle. Make sure you replace nutrients that are lost through water like electrolytes after strenuous activity or anything that causes major water loss. Your body will thank you. STAY STRONG!