Underwater Girl Portrait

An immediate obstacle to human activity under water is the fact that human lungs cannot naturally function in this environment. Unlike the gills of fish, human lungs are adapted to the exchange of gases at atmospheric pressure, not liquids. Aside from simply having insufficient musculature to rapidly move water in and out of the lungs, a more significant problem for all air breathing animals, such as mammals and birds, is that water contains so little dissolved oxygen compared with atmospheric air. Air is around 21% O2; water typically is less than 0.001% dissolved oxygen.
The density of water also causes problems that increase dramatically with depth. The atmospheric pressure at the surface is 14.7 PSI or around 100 kPa. A comparable water pressure occurs at a depth of only 10 m (33 ft.) (9.8 m (32 ft) for sea water.) Thus, at about 10 m below the surface the water exerts twice the pressure (2 atmospheres or 200 kPa) on the body as air at surface level.
For solid objects like our bones and muscles, this added pressure is not much of a problem; but it is a problem for any air-filled spaces like the mouth, ears, paranasal sinuses and lungs. This is because the air in those spaces reduces in volume when under pressure and so does not provide those spaces with support from the higher outside pressure. Even at a depth of 8 feet (2.5 m) underwater, an inability to equalize air pressure in the middle ear with outside water pressure can cause pain, and the tympanic membrane can rupture at depths under 10 ft (3 m). The danger of pressure damage is greatest in shallow water because the rate of pressure change is greatest at the surface of the water. For example the pressure increase between the surface and 10 m (33 ft) is 100% (100 kPa to 200 kPa), but the pressure increase from 30 m (100 ft) to 40 m (130 ft) is only 25% (400 kPa to 500 kPa).
Any object immersed in water is provided with a buoyant force that counters the force of gravity, appearing to make the object less heavy. If the overall density of the object exceeds the density of water, the object sinks. If the overall density is less than the density of water, the object rises until it floats on the surface.