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.
When ambient air pressure is higher (for a given temperature), the density of the air will be higher, so buoyancy will be increased - just slightly. There is pressure above you pushing down and trying to increase your weight, but there is just as much pressure below you pushing up trying to decrease your weight. So they ALMOST balance out. But, there's actually slightly more air pressure below you than above you, so the overall total effect is that your weight actually gets decreased. This is the upward thrust you speak of, otherwise known as buoyancy. It takes the pressure above you into account. Thanks , for appropriate explanation.