Peter Hackett, a well known researcher in the field of altitude sickness, suggests in this interview that you actually want to be dehydrated at altitude.
There's evidence that the people who do best at altitude are dehydrated. That is the body resets the serum of molality level which has to do with the water balance. And the body, for some reason, prefers to be dry at high altitude. My own thinking is that this is good for the body because it keeps the brain a little bit drier and softer. So that if it does start to accumulate a little water or get a little swelling, it can be tolerated better.
Wow. Conventional wisdom that hikers / climbers digest elsewhere may need to be altered. I suppose it's time to read up more on this issue. Maybe I've been drinking too much water in the Sierra!