Another way to know if the sub is sunk is to look in the tactical screen, the submarine red tracking lines, disapear automatically.
But I have to say that the oil slick is a nice feature and it is nicely implemented.
Just a comment about a previous statement from me regarding the possibility of having information about the convoy Base course and speed.
I have just discovered that information (BC and SPD) is already displayed in the „Contact station“ however apparently it does not update if the convoy maneuvers.
I have just discovered this yesterday and I need to take a look again. I belive the information about own ship also does not update, but in any case I am happy that the developers have already consider this and probably this will be fully operational at a later stage.