Since you would be aware it was happening, and probably struggling against it, my bet is you would over-exert (trying like the dickens to kick up) to the point of getting a CO2 narc, inducing panic, and bringing on irrational behavior like spitting out your reg. Bottom line, you would either black out in a panic from excessive CO2 and not enough O2, or spit your reg, but either way you would drown and your body would likely never be found. It would be an ugly and, as others have said above, completely un-necessary death caused by stupidity, carelessness, or both.
