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2)HULL CAPACITY. Hulls are special structures in a submarine to hold water in them which help the submarine to maintain buoyancy underwater. The deepest a submarine can go is about 3 km underwater(I dunno if modern technology has surpassed that level or whatever).
#2. redargon. http://en.wikipedia.org/wiki/Submarines. It appears, the deepest is about 1400m. I only found one torpedo that can be used at 800m so far, others are limited to about 400m. I'm not sure of the limitations of actually firing one, but that is the design depth before the torpedo is crushed.