it will be a painless death as you run out of oxygen.
You will have a slight head ache at first, followed by a sense of euphoria.
then you will sleep.
then you will die.
OR you can pull your pistol from your locked bunkside table, load the thing up and each man may shoot himself as he chooses.
could you imagine being the last man? what a lonely dreadful feeling it would be to realize your there 80m under the sea, in a boat that will never surface again, with your friends laying dead all around you and its your turn to commit suicide.
of course... 80 meters of water... thats like 260 feet. would the escape lungs be effective at such a depth.
talk about a procedure! flood the entire U-boat and then open the hatches and each man ascends slowly to the top.