[yetanotherloser]

The advantage of testing pressure vessels with water is that the failure mode is usually less bad than gas (because it doesn't expand through any breach like gas does).

Unfortunately a sub is the opposite of a pressure tank (the pressure is on the outside) so you need a pressure bottle bigger than the sub to test it in. I don't think these are common or easy to come by.

[jacquesm]

I've been in the reverse, an autoclave large enough to hold an entire spacecraft. That was already engineering on a scale that defied my imagination considerably, doing the same at the level where an entire sub could be pressurized to 400 atmospheres is engineering on a different plane. I don't think you could do this any cheaper/easier than just strapping it to a tether and dropping it overboard in a very deep part of the ocean, then winch it back in to see if it survived.

[yetanotherloser]

Gosh, that's one big autoclave. As you say, a sub tank would be even more of a monster bit of engineering. They do exist for more normal depths but I'm not sure if there are civilian / rentable sub test tanks for this kind of ultra deep stuff.

[tim333]

I guess in practice for something like the titanic sub, you could program it somehow to dive unmanned to some depth deeper than normal use and then resurface. Presumably with some radio beacon so you can find it when it does. Or a long cable connected to monitoring devices. If it didn't come back you'd have lost a sub but not lives.

[jacquesm]

I'm not even sure they exist at all for that depth.