After the attack on the convoy they resurface and find out that one of the ships they torpedoed, is still afloat, even though it's burning and it's almost broken in half. It looks obviously totally crippled. In order to sink that tanker more quickly, they fire another torpedo at it.
However, the rest of the fleet already moved on, they are no longer in danger, they could just have used their deck gun. In fact, U-boats in real life preferred using deck guns against targets not armed well enough to pose them a danger, because of the limited supply of torpedoes they could carry.