Just using a LOT of water is a bad idea (unless you eat a good amount, with salt and other nutrients around that time). Because as we all know, this can lead to hyponatremia (at the risk of stating the obvious: you sweat out a lot of salt with the water you lose. Only replacing the water dilutes your blood (serum).
As altitude  increases, the air becomes less dense because of the smaller amount  of air above that altitude. There is a corresponding decrease in  pressure with an increase in altitude. Eventually you reach a point  without air, and the pressure decreases to zero. That is space. + 91 others found this useful.
The air column below you will not contribute to the pressure on you. All that matters is the weight of the air column above you.But there is always less air above us than was present at a lower altitude. So the pressure decreases with increase in altitude. About density variation with altitude:- The density of air depends on both temperature and pressure through the equation of state (Remember how the pressure, volume and temperature are related in thermodynamics).