Is there a such thing as too much water?

The kidneys of a healthy adult can process fifteen liters of water a day! You are unlikely to suffer from water intoxication, even if you drink a lot of water, as long as you drink over time as opposed to intaking an enormous volume at one time. As a general guideline, most adults need about three quarts of fluid each day. Much of that water comes from food, so 8-12 eight ounce glasses a day is a common recommended intake. You may need more water if the weather is very warm or very dry, if you are exercising, or if you are taking certain medications. The bottom line is this: it's possible to drink too much water, but unless you are running a marathon or an infant, water intoxication is a very uncommon condition.

You would have to drink a lot of water though for it to really damage your body.

There is actually a problem with drinking too much water. You can develop Hyponatremia, which means your salt levels are too low and severe cases of hyponatremia can lead to water intoxication, an illness whose symptoms include headache, fatigue, nausea, vomiting, frequent urination and mental disorientation. So yes, it is a good idea to drink lots of water, but chugging gallons of it during the day might not be the best idea. I don't think you would be retaining water by drinking this much, if you are not losing it is probably for other reasons, but just be careful about chugging the water and make sure that your diet is balanced.

Yep, this. I learned about it paramedic/nursing school, although it is more common in children, than adults. Drinking gallons is a bit excessive and I don't even understand why you would.