[BigBadVuk]

Most of the subs casualties are by implosion...U see a subs are build strong enought to withsand a lot of presure (btqw cylinder is a geometrical body with very high pressure tolerance)but at greater depths all bulkheads will colapsed all together in miliseconds interval becose of the so called domino effects: they r already well beyond their calculated stress and it is enought to just 1 of them colapse( i assumed that there is 1 or 2 ruptured compartments and that they r flooded so the pressure on the bulkheads are same as outside of hull) and cause sudden and agressive change of pressure to second 1 and then that one will colapse and so one...But like i sad usualy it takes miliseconds to happen and usualy boat is splited in 2-3 part(look at images of Trasher and Scorpion wrecks for example).If hull burst before bulkhead then it will be filled with water in seconds and u will be dead before u realise what happened becose your inner pressure is much smaller then outside.Your lungs,heart,stomack will colapse in a splitsecond and u will be dead.
If u were the part of crew u will be sitting in dark,in some unusual upsidedown position all around of u will be that rumbling-creeking nose of hull under pressure and then "snap" in 1 milisecond u r dead..Very fast and very merciful death compared to drawning...