"Am I to understand that values in the .zon files are (ugh) in metric?"
Yep, Ubisoft is in Romania, one of them heathen European countries that think they're too fashionable to use good old Christian feet and yards.
Not a huge problem, meter times 3.3 = feet, yard times 1.1 = meter. Approximately, close enough for the military anyway.
Several other factors though, if your ingame systems screen shows hull damage, 200 feet minus 10% = 180 feet as the new crush depth.
In the default S class;
Hit points = 500
Crash Depth = 122
Crash Speed = 2.0
If you look at SKWAS's notes, most seem to be correct (guy who wrote it wasn't an Ubisoft employee so he's wrong sometimes, but not often), and he notes "crash speed" is the amount of hit points you lose per second below crush depth. 2 per second is 120 per minute, so 4 minutes below 400 feet would use up all the 500 hit points and you're dead. Further complication, below 400 feet for 1 minute, got 500 hit points, lose 120, you're somewhere around 25% hull damage, new crush depth is 400 minus 25 %, 300 feet. Rising to 350 won't do you any good, have to get above 300, but by the time you get there you've taken more damage, now it's 250 feet, um time for an emergency blow? For that reason I changed all my subs to a crash speed of 0.1, so I lose 1 hit point every 10 seconds instead.
Kind of a moot question for creaks and groans though, all my testing back then indicated that the four .wav files play randomly in random order, and pay no attention to actual depth or depth changes. Would have been nice if they programmed it to creak and groan louder and louder as you went deeper, to give fair warning when you were approaching the limit, but they didn't.