Effect of pressure on gas solubility in liquids
The pressure underwater increases by roughly 1 bar every 10 meters, that is, ambient pressure is doubled every 10 meters compared to standard atmospheric pressure.
When you breathe air underwater using a SCUBA tank, your lungs expand against the surrounding ambient pressure in order to increase their volume. As the lungs expand they fill with gas which props up the lungs (balances pressure within the lungs with the surrounding pressure). The SCUBA device adjusts the pressure of the delivered air so that it balances the ambient pressure. The partial pressure of the component gases in the gas mixture inhaled from the SCUBA tank is not the same at the water surface and when a diver is submerged. That means the pressure of the air in the diver's lungs is not ~1 bar, it can be far greater, within limitations imposed by the potential toxicity of high blood concentrations of oxygen and nitrogen.