Correct me please, 'cause I have been living under Hollywood special efects for a while, but a submarine doesn't behave as an aicraft. A fracture in the pressure hull is not instantly fatal. It's seriousness depends on depth, on metal strenght and placement. Water would be forced in at great pressure, but it is a fissure in the metal skin. With time, pressure and water, it will grow bigger, but if water is pumped out of the boat as fast as it gets in, and if the boat is brought to a safe depth, it will survive.
This will illustrate what I mean>
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