[Mikey_Wolf]

Its basic physics, even if the US subs and the U-Boats had the exact same steel the exact same thickness, which one will dive deeper?

The U-Boat naturally, because its smaller which means when it submerges there is a smaller hole in the water and the overall area of force exerted on a U-Boat is smaller than that of the US sub.

Though I would not be surprised if the German U-Boats were thicker pressure hulled anyway, because the Germans were extremely adept at Submarine warfare they learnt a lot of lessons from the Great War.