i personally cannot believe that anyone voted no.
Not that submarine versus submarine combat was a major operational consideration during ww2 it was at least a threat.
Im not sure of the numbers but there were a number of british and german submarines cought on the surface in the atlantic campaign while recharging batteries or whatever who were torpedoed without warning.
It would be very frightening to be on the watch tower in your Type VII making your way through the bay of biscay out of lorient or whatever and suddenly you spot the visible trail of a torpedo heading right for you!
ALAAAAARM!!!! HARD TO PORT!!!
this WAS a constant thing that watch crews had to be aware of in some areas of the Atlantic.