It's pretty much the exact opposite of designing a spacecraft.

Then let me amend what I wrote: It's as difficult as designing a spacecraft, because you reach the limits of what materials that can be used can offer.

Well, once you are in space the forces are pretty tame until you get hit by something. But the ocean never rests from trying to kill you, even the smallest material defect can cause an implosion. Personally I think designing for a delta of 400 atmospheres in compression is much, much harder than for 1 atmosphere in tension.

But it's withstanding pressure(called external pressure, yesterday I learned), so can't they just have a clean, stepped flange to mate and to prevent slipping off, plus a simple lock to prevent unintended disengagement?