I've waited for hours of game time for a ship to sink. I'd guess that when I see the lifeboats paddling away that she's sinking, but I saw one ship sail on for hours afterwards. In my previous patrol I got credit for a sinking in my logbook and hours later it was still afloat. After I broke off from the convoy and sailed away, she was still sitting there. I fired a few deck gun rounds at it just to put it under. I didn't want the British to "salvage" it.
