I remember reading somewhere that one torpedoed shipwreck was just left floating once the sailors had been evacuated, then a U-boat pounded the wreck with a deck gun for tens of rounds, and then just left the wreck, that was still floating. Historically speaking it would be ok just to assess the damage done and make an assumption that the ship can't be sailed again and report to BdU a ship most likely sunk, eventually.
Of course the game engine doesn't simulate this, so if you would leave a ship waiting for it to sink and it didn't, you would have to modify the save game files to your liking, if you still wanted the renown that should be coming to you. Too much trouble IMO, just leave the ship, hope for best, and wait for the VII or IX - then you'll be able to blow wrecks to smithereens with that one extra torpedo. There's lots of fish in the seas that shipwreck came from.
