Theoretically it would be easy to construct a sub which would dive without the need of pumps: a sort of mechanical device that opens the ballasts to let the water in. Problem is: Such a construction would be very frigid and subject to easy damage in combat, so it's better to have pumps that are well-shielded against explosions etc. If your pumps or ballasts are gone your are in a liquid grave...
So in your situation, the pumps must reload to apply pressure and fill the ballasts with water removing the air which applies forward force and impedes any diving. You need to be that patient, in reallity it lasts more than 2 minutes I imagine which means skippers do emergency blows really at last ressort because any time on the surface is deadly.
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