Unconscious in 20 seconds
Under water, the semipressure of Oxygen in your blood is lower than the ambient semipressure, consequently it remains in your blood until consumed. In addition, you have the air in your lungs. For depressurisation, firstly you don't have air in your lungs at a sufficient pressure to absorb oxygen and secondly if the ambient pressure drops to below the semipressure of oxy-hemoglobin then the oxygen in your blood is breathed out.
At least that is my memory of University physics...