In SH games it's largely zone dependant. Torpedoes in the bow portion of the ship seems to be unable to sink it from flooding.
Damage the stern portion of the ships with 2-3 torpedoes and they will eventually go down; if you save and reload you'll have a crippled unsinkable ship that requires more damage to be put out for good.
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...Sinking deeper into the cold, dark oceans of life 
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