Klaus said:But if the sea floor pressure can smash a human body into jelly, why not a zombie one?
Because the whole "smash a human body into jelly" thing is a vast overstatement.
If you are trying to keep lungs full of air, deep-sea pressures will crush your ribcage, yes. And any air-filled cavities in a human body are a problem. However, a zombie doesn't need to keep any cavities filled with air. he can let his lungs or other cavities fill with uncrushable water.
From there, your body is mostly a big bag of guess what? Salt water! It'll compress only a little bit, under very high pressure - so for a living human being, the extreme depth is a problem because of stress on the skull (a cracked skull won't stop the zombie). Eventually, pressure will even shrink the brain somewhat - just enough to to muss up the synapse length between neurons, and that's a problem. Last time I checked, though, zombies don't care so much about being smart
