When a submarine implodes, a variety of fairly ugly things will happen to the crew. If we assume that a pressure hull implodes at 2000 feet (~60 atmospheres), the pressure will increase from 14.7 to about 875 PSI almost instantly. In the parts of the submarine that have volumes of trapped air, it would be like being inside a diesel engine cylinder when begins its compression stroke.
Anything flammable would burst into flames until a huge wall of water slams into the area and snuffs it out again. The impact of the water would cause significant injury to anyone unlucky enough to still be alive and there would be no time to suffer the effects of oxygen poisoning or anything else.
As others have stated, most human tissues are fluid-filled and are for the most part, incompressible. Human lungs and sinuses would be crushed instantly and the immense shock would render them unconscious immediately. Of greater concern would be the surge of incoming seawater, bulkheads, decks, heavy equipment, motors and other random bits of equipment being slammed into the crew at high velocity.
Essentially, the crew would be killed several times over in less than a blink of an eye.
According to several articles about the loss of the Scorpion, when the pressure hull failed, water entering the hull was moving at supersonic speeds and had filled the entire space in within 100 milliseconds.
The collapse produced with a force equivalent to 6000 kg of TNT.
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