same here, had a few occasions where a warship took a shot at me, one hit results in a bulkhead + hull completely destroyed in a compartment -> death
had one interesting case where i landed on the sea floor at 360 feet. the boat kept colliding, resulting in slight damages all throughout. slowly crunched all compartments (as said in a manual/readme/on this thread somewhere), nothing i could do about it. death came after half an hour.
So I guess there is no hope as soon as you take even one shell?
edit: to clarify some details on two deaths:
-Porpoise vs taskforce, tried to sneak in on the surface, escort came very close and i was lucky enough to be already on my way down before they opened fire. a little DC damage which was repaired. a short while later at periscope depth inside the taskforce, they fired on my periscope, I took a hit to the conning tower and sunk like a brick, unable to control the descent at any point. dead
-Gato on the surface, keeping an eye on a convoy as it went past me on the horizion (all ammo already expended). went into 128x TC (won't be making that stupid mistake again, lol), took incoming shells as they spotted me, ordered crashdive as they hit me. engine room bulkhead and hull with full red bars, uncontrollable sinking. hit the bottom at 360 feet and got slowly eaten up by collision damages.
conclusion so far after 3 encounters with escorts: depthcharge damage / damage model seems to work very nicely but as soon as you take a shell you are dead
very intense, very scary, I am really getting afraid of warships
(although I was not too happy about being shelled at P-depth, I don't suppose it's possible to code them into ramming instead of shooting? And I was secretly hoping to be able to save my boat with repairs while sitting on the bottom but no such luck. I didn't stand a chance at all, lol)