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.
So the temperature of the air near the earth's surface is greater and the value decreases with increase in altitude. This is why the speed of sound (which has a direct relation with temperature) decreases with altitude. About pressure variation with altitude:-The pressure of air is nothing but the weight of air column over a given surface.