US boats show distinctive oil canning in their free flooding areas, U-boats don't seem to show much if any.
What was the typical thickness of the Gato/Balao class skins?
BROAD GENERALIZATION ALERT!!
We yanks tend to be thinner skinned than those across the pond. Perhaps our boats were as well?
EDIT:
Gallery of U-505 pics during the original transit to Chicago. Note the obvious bumping/collision damage with a dearth of oil-canning.
However:
U-995 in Lahoe shows a little bit of curvature in bright light, which is not visible in other pics in cloudier conditions.