Deep Sea Diving: Significance of Partial Pressure

SCUBA is overwhelmingly the most common underwater breathing equipment used by recreational divers. The fundamental item of diving equipment used by divers is underwater breathing apparatus, such as scuba equipment, and surface supplied diving equipment.
Divers who breathe from an apparatus that delivers gas at the same pressure as that of the surrounding water need not return to the surface to breathe and can remain at depth for prolonged periods. Pressure is the first and most important factor that must be overcome for humans to be able to breathe underwater. Water is much denser than air. When you dive to the bottom of a pool you immediately feel pressure on your ears. This is because all the water above you is pressing down on you and squeezing the air inside your ears into a smaller volume. You do not experience this when going down a short flight of stairs because air is not as dense. The extra air that presses down on you at the bottom of the stairs does not weigh enough for you to feel it.
An easy way to think of partial pressure in scuba diving is to consider it a measurement of the concentration of a particular gas in a diver's mixture of breathing gasses. As the concentration of a particular gas in a diver's breathing gas mixture increases, the physiological and psychological effects of that gas may increase or change. For example, extremely high partial pressures of oxygen may be toxic (oxygen toxicity) and very high concentrations of some gases, such as nitrogen, may cause narcosis.
When you swim down underwater, the high-density water all around you presses in from all sides. This means that if you were to try to breathe through a hose from the surface of the water, your lungs would need to push all that water weight outwards to make room for the air. This is impossible and that is where SCUBA comes in.
When you breathe from a SCUBA tank, you breathe air at the same pressure as the surrounding water. This means that, although the water presses in on you, the air you breathe presses back out with the same force. This makes it easy to breathe. Breathing pressurized air under water is the same as you are breathing now. The air around you is pressing in on you from all sides, but it is not hard to breathe because the air you are breathing is pushing out with the same pressure as the air around you.