What seems to be getting missed here is that all this plumbing and tanks was protected from a direct pressure wave by the U-boat\'s second skin. Everything which was outside the pressure hull was still inside the outside hull. So what really happens is that the whole U-boat gets shaken up and to some extent bent by the uneven application of shock waves. Ameliorating this shock damage through appropriate anti-shock design is an engineering discipline which was just getting started in 1943 with the design of shock mountings for periscopes for example.

The problem with the hydraulic system of the XXI is a long story and much more complex than mounting some hydraulic lines outside of the pressure hull. The XXI only got a hydraulic system because there was not enough copper wire around to make electric motors, which were used to do most things on previous U-boat designs. The shipyards knew little about hydraulics, so there were a lot of problems not only in design, but also in building. One of the reasons it took almost a year to get the XXI into service after the first one was launched was due to endless problems with the hydraulic system. As it turned out, there were some vital hydraulic systems outside of the pressure hull which could not be reached for repairs, so the issue was more than just a few hydraulic lines. The air systems were designed with redundancy, but apparently the hydraulic system was not.

Regards,
SuperKraut