Drinking to much water basicly causes you to drown in your own body it can be a problem for small children in swimming pools when they swallow alot of water.For the most part though is people drink do not enough water you can also get it in tea and other beverages like fruit juice, sodas are not a good choice and I believe the that sugary drinks make you more thirsty.
If you are thirsty and want water you are already way too dehydrated you must drink it enough before you actually get thirsty.A good way to tell if you need more water is when you go to the bathroom and you urine is dark colored the darker the worse dehydrated you are it should be a very light pale yellow or even better clear just like water anything darker is a sign of dehydration.
At high altitudes you need to drink copious amount of water ,especially at 10000 feet drink,drink, drink and then drink some more it helps you adjust to the elevation sooner by doing a kind of kidney dialisis turning more white cells to red which in turn gets more oxygen to your body which you need as there is less oxygen up there.You can tell you are getting enough for that if the fiirst few nights you need to get up at least twice a night to relieve your self.
I worked at a ski area for 11 years at 9500 ft then worked in summers at a lodge at 10000 ft for 8 years.So I did a little home work.Personaly Im not far from a bottle of water or ice green tea ever, it's by my bed also but living in a real low humidity it sucks the moisture right out of your body.
Also a good way to prevent kidney stones is to drink alot of water it keeps your kidneys flushed my husband used to get them and when I told him 12 years ago he just needed more water they disappeared has had not one since.
Sorry to be so long winded buy it's kind of a passion of mine I could go on and on..Especially high altitude sickness.
I actually have come up with some supplements that can get you adjusted to high elevation very quick it worked on every kid that came to work at the lodge.
If interested I will post.