The underwater environment is vastly different from the above water environment. The body in the underwater environment must be able to adapt to much greater pressure changes and more rapid pressure changes than encountered in performing military aviation, high altitude parachuting and high altitude mountaineering activities.
The underwater environment is not an atmospheric environment and is a hostile physiological environment. Extended underwater activities beyond breath holding require use of some sort of supplied breathing provision.
Diving techniques include free (breath-hold) diving, snorkeling, SCUBA (Self Contained Underwater Breathing Apparatus) diving, surface supplied air and mixed-gas bounce diving, and saturation diving. Free diving is the only underwater activity lacking use of some sort of supplied breathing provision (life support).
Under certain circumstances, any type of diving may result in decompression sickness, barotraumas (tissue damage due to increased pressure), and pulmonary overinflation syndromes (trapping of gases in lungs, with potential for rupture of alveoli leading to arterial gas emboli).
Human performance capacity to physiologically adapt (cope) to the at all times hostile underwater environment is much more than the simple imminent danger of drowning should any mishap of occur with equipment or as a result of inclement diving conditions.
The perplexing physiological adaptation (acclimatization) includes individual tolerance to increased underwater pressure varies significantly as depths increase beyond 33 feet of sea water (2 atmospheres-the combination of atmosphere and water weight at that depth). At the greatest depths of the ocean (approximately 36.000 feet), the pressure is more than seven tons per square inch (1,100 atm)