There is one thing you could try if you can get in the right position that is.
If you can find an unarmed Merchant and sail right next to it, the enemy will be reluctant to fire on you because of the merchant.
I found this out by chance, I sailed next to a damaged merchant and the DD's just circled me not firing, I then realised it was because of the merchant.
Again a similar incident, I was playing as a ship and a DD rammed me but got stuck next to me, the DD's again would not fire till the stuck DD eventually separated from me.
Going back to SH2, I once hid right next to a sunken ship, it was in shallow water by the way but the DD's went right by looking for me and I got away, but this was in SH2 so obviously things are slightly different in SH4 but food for thought.
Col.
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Silent and Violent
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