Well, they tell you
that, by informing you that you're "diving too deep".
"Oh, and skipper, you know how the boat is sinking due to the flooding, even though no one's bothered to inform you? Well, now we're five seconds away from crush depth. Just thought you'd like to know"
.
You gotta admit, they've got a certain charm
.
Another nice feature for the "Easy" difficulty level would be for the game to tell you what your crush depth was at any given time. Like, "U-538 Type VII-C
Hull Integrity: 46%
Crush Depth: 95 m"