I`ve read some explanations from physicists about this issue and a number of things happen depending on the situation. For one thing, if you try to hold your breath, you`re really just killing yourself quicker- the air inside your lungs will expand until your lungs rupture and release air into the circulatory system.. The water in the eyes and mouth would boil immediately, along with water in your muscles and skin tissue, causing bloating, but not enough to break the skin. The nitrogen in the blood would cause bubbles which is apparently very painful.
The effect of the lungs works in reverse since the gas is expanding, so the oxygen in the blood stream would pour out instead of going through. Once all this is done, the brain will start to suffocate and you`d begin to lose vision and control of your motor functions, not to mention the impaired judgement. Your nose and mouth would be near freezing at least from the effects of evaporation- and you`d probably have a terrible sunburn (assuming this is the case?). Then you pass out after convulsing around a bit.
The brain and heart still work, however, and up until about 90 seconds it`s possible to resuscitate someone with pressurized oxygen, though the blindness might stick around for a while. After this point, the blood pressure drops low enough that it will begin to boil and the heart will stop beating.
I READ THIS, I AM NOT A PHYSICIST.