Flooding
Flooding resulted in sea water mixing with battery acid = chlorine gas
Chlorine gas was deadly and very much feared.
U-boats were not divided into many little water tight departments like shown on the crew diagram. If I recall correctly, there were only two water tight bulkheads in the U-boats; one just forward of the control room and one aft. These could be closed but massive uncontrolled flooding fore or aft would create a major trim problem.
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