[seafarer]

High velocity rounds tend to disintegrate on hitting water and rarely penetrate more then a couple or few feet. Even 20mm cannon rounds won't penetrate very far.

At a keel depth of of 30m, the upper deck of even a even a type IX is what, 15-20m below water, so no way an aircraft machine gun or cannon round would penetrate that much water. Pieces of rounds may float down and tink against the hull, but in the real world, no .303 or.50 or 20mm round would have caused any damage.