http://en.wikipedia.org/wiki/Submarine_escape_training_facil...
So it must be fairly survivable.... Maybe it's staying for a long time at that pressure that causes the problem?
http://www.adventuresunderthesea.com/scuba-class-information...
The deeper you go, the less time you can stay there before you have to control your ascent.
Even a mild case of the bends can be excruciating, not something you can tough out: http://en.wikipedia.org/wiki/Decompression_sickness
If you want to have a go, you can go to Nemo33 in Belgium still.
Cf : http://www.worldnavalships.com/forums/showthread.php?t=3787 http://www.subescapetraining.org/History1.html
The longer you breathe at pressure, the more time there is for gas (nitrogen, mainly) to saturate your system (until you are at equilibrium, anyway).
Release the pressure, and it's like opening a can of soda... all that dissolved gas can't stay dissolved any longer.
It takes time to get out of your system as you reduce pressure, so a gradual decompression is the only safe way out once you are saturated.
In a typical submarine, air pressure is maintained at standard atmosphere, relying on structural integrity to keep all that pressure from crushing the ship. That means they can surface any time without risk.
Another neat thing when diving is that if you are down at 30 meters, you can take a breath from your regulator, spit it out, open your airway gently (say "AAAAH" quietly to yourslef basically) and then ascend, you feel like you have an endless supply of air, because that breath you took is expanding on your way up. (That's part of recreational diving.. you can ascend from 30 meters on a breath of air and not get stressed out about it)