I've had ships destroyed without them actually sinking. The only time they plummet like that is when you blow away all their HP, which isn't strictly nescesary.
Quote:
I've done a keel shot ( don't ask me how because it was with impact switched on)
the ship took water and was pointing down in the bow.
Now that same ship didn't want go down with 4 hits!
repeat 4 hits!
The next patrol 2 transports.
1 torp was enough for one of them the other sailed slowly away.
Then i found a task force.
Hit a battleship with 2 hits and still nothing wrong.
Then the battlecruiser behind it completely exploded with one luck shot 
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Sounds about right to me.
* Oh and iirc capsizing was rather a rare thing to happen, not realistic to have this happen all of the time at all.