http://uboat.net/allies/ships/uss_bogue.htm tells about U-758 which "put up heavy antiaircraft fire from her new quadruple 20mm cannons. She eventually submerged, withstood depth charge attacks by destroyer Clemson, and escaped despite a flooded compartment." Sot it was possible! that still leaves the question as to whether this could be modelled under SH3. Perhaps messing with the damage settings, but that might give you an invincible sub.