Here are some of the differences between air and water pressure: Water is much heavier than air. A cubic foot of air weighs 1/12 pound. A cubic foot of fresh water weighs 62.4 pounds and a cubic foot of sea water weighs 64 pounds. Air is a mixture of gases, principally oxygen (O2, 21 percent of the air by volume) and nitrogen (N2, 78 percent by volume). Each gas exerts its own independent pressure, the sum of which equals the total air pressure (Dalton's law). Unlike water, air (and any other gas or mixture of gases) is compressible; the greater the pressure exerted, the more tightly packed together are the individual gas molecules. Regardless of the air pressure, however, water molecules are much more tightly packed together than air molecules. Compared to air at sea level pressure (1 atm.), water is about 800 times denser. Just as air has weight and exerts pressure on all sides of an object in the atmosphere, water exerts pressure around any object immersed in it. We can push water out of the way because its weight is distributed on all sides and the molecules can be easily moved. The resistance we feel under water reflects the extreme density of water (compared to air), and the fact that it takes time for water molecules to move out of the way. Water pressure, like air pressure, is a function of weight; the deeper one goes the greater the surrounding water pressure.
