guys, sea pressure is somewhere around 50psi/inch per 100 ft when submerged (depending on salinity, temperature and all of that crap) an actual submarine will implode (from what I understand, I haven't experienced the implosion part just the crush depth). Typically you have test depth which is a safe normal operating depth, then "crush" depth, this is where the boat should implode, I know it's a wild ride when you go that deep. Seeing how the boat needs to contract and expand due to sea pressure, everything is built so it is floating, the deck plates, and everythign mounted to them. As you go deeper, you hear creaks and pops, but when really deep these start to get to be pretty huge and can scare the **** right out of you.. it's definitely a fun ride...

and seeing how crush depth is more of a design number, the boats usually can exceed this depth a bit (I kinda missed ordered depth one time and found out)

as for the real thing, you'd get shaft seals leaking (more than usual) and everything else that penetrates the hull would start coming loose and leaking, the flooding would pretty much take care of the rest, it would be a very quick ride from that point to the implosion point I would think...