Don't worry about it. I think many of the complaints of subs come from players that don't quite understand what NF was trying to emulate with the mechanics they used in the game. Battleship players, at least when subs were first introduced, has this idea that anything under a certain size, should be simply destroyed with one hit from their guns. Yet, when subs came around, suddenly they (meaning the battleships) were finding themselves being destroyed by something they both couldn't see coming half the time, and couldn't destroy  in one hit like they were accustomed to.   I distinctly remember one post I caused, when my SS2 fired at and sunk a BB5 (back when BB6 weren't in the game) because the BB5 had no bulge.  (before subs were included, very few BB players packed bulge.) It didn't take long for players to adapt, but every so often you see posts from newer players that honestly just don't understand.

No shame in not understanding.  However, for what it's worth, your point about not being able to critical dive when seriously damaged?  That is actually a good point. I wouldn't say until fully repaired, but I would agree that a critical dive would be dangerous for a submarine that has suffered serious damage.

Fact is, the deeper a submarine goes, the more pressure there is on the hull. If, for example, a Ww2 submarine had a leak and submerged, it might not have a problem. However if it went to its critical (crash dive) depth, what might be a minor leak at other depths, would become a jet of pressurized water.

That jet of water would have NO problems cutting through flesh, or in some cases steel. (google the following for better details:  water jet cutting)  I see no reason then, why a submarine that has suffered critical damage, should still be able to endanger itself with a critical dive.

Gross fact incoming:

One more thing to note, just a random fact of the day. When a submarine passes its crush depth and implodes, the crew don't suddenly drown, and aren't crushed by the flood of water.  They're incinerated.  The sudden pressure of water flooding into the boat causes the air inside the boat to compress so fast that it produces an insane amount of heat, turning all crew inside to ash in a matter of miliseconds.