Funny, last night I had taken damage. I decided to check out the damage % screen. It showed 15%. I though, ok, cool I keep sailing. Later on, I was kept down by some DD. I went to the the red line on the depth meter. I forget the depth. After about 5 minutes at the deepest the boat was able to go, per the red line, I start hearing the usual, this is damaged that is damaged blah blah. I briefly see water start leaking and then the instant death screen.
I'm guessing that if the hull has some damage....best to watch how deep you go. Keep an eye on that meter gauge. If she is climbing...you be too deep