Physical Considerations
Water is much more dense than air ( 1.02-1.1g/cm3 as opposed to .00125g/cm3
Water is not very compressible but air is very compressible by comparison
At a depth of 10 meters (33ft), gas is under two atmospheres of pressure-- one Atmosphere of pressure exerted by the atmosphere and one by the water column
Partial pressure = the % of gas in a mixture x the atmospheric pressure, so at 2 ATM the partial pressure of oxygen is 320 mm Hg, not the 160 mm Hg it would be at the surface