My layperson understanding is that with space you only have to have something robust enough to keep the atmospheric pressure in (as well as other considerations of course) which allows for less robust materials. For deep sea exploration you need something robust enough to keep the water pressure out.

For additional info: 10 metres of water depth is approximately equivalent to 1 atmosphere's worth of pressure (ATM) - so 50 metres is 5ATM and so on and so forth. So theoretically a submarine would have to combat hundreds of ATMs of pressure, whereas a space craft only has to combat at most a couple of ATM.

In the ISS a minor hole can be patched pretty easily and quickly as it's a slow leak of air out, however if a leak occurs in a submarine the results can be explosive and deadly.

Not an Apple fanboy in the slightest but one could argue that the fact it was so difficult (without knowing ALL the security details) is a feature not a bug.