[UKönig]

In my experience, if you do not stick around until some kind of confirmation has been received, then no, you do not get credit for the kill.
Because you didn't stick around to observe the sinking, as far as you know, the ship could be taken in tow and saved. True, it'll be down for repairs for a while, but not destroyed.
The only thing you can do in that case is to wait around until conditions improve so that you can surface and finish it off with the deck gun. That part becomes the most trying on the patience of the captain involved...