Jonathan
Im surprised by your east coast experience... how did he locate you?
As a general rule on those realism settings when a destroyer is sighted or warship detected you want to slow to either ahead slow or all stop, rig for silent running by pressing the "Z" key and wait for the destroyer to leave the area. go deeper if possible.
just out of curiosity - what did you do when the destroyer was detected?
sounds like your situation should have been survivable - especially in stock.
Loading a save would not be considered "cheap" as you put it, especially if you are still in the stages of learning to play the game really well.
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