If you hit a ship and it doesn't sink... like in just the screws have stopped, hide under it. The escorts will run circles around it droppingf DCs. After awhile they just leave, but you should wait longer. Then when they're 20kms away you can finish it off. But becareful, later on in the war, they might have deckguns to give you a hard time.... stay submerged to be safe.
