In my experience this sudden death you experienced usually happens when the game is simulating stormy weather during high time compression and you happen upon a warship or armed merchant - the time compression wont drop until your actually under attack or your watch crew actually sees the enemy and it might be too late by then.
on anther note... did you actually see the enemy ship that got you or did you just suddenly die?
if this is the case... you wandered into a mine field at high time compression - big no no because you just suddenly get a death screen usually.
about saving a dieing boat?
once i struck a mine which caused signifigant damage to the bow quarters wounding several men and killing 2 or 3 of them.
the water was only about 60m deep.
my boat sank all the way to the sea floor and struck bottom causing some more damage.
i placed the damage control team into full swing, eventually most of the water was pumped out and i managed to blow every ounce of compressed air to bring it to the surface.
i cruised home with the bow almost completely submerged as the last bit of water would not pump out.
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