Actually escorts
could hear the torpedoes as they made their runs in.
During a U-Boat torpedo attack a sonarman picked up the distinctive hiss of an approaching fish.
"I ran out on the bridge to prepare for an explosion. I looked over the side in time to see the torpedo streak on by about 10 feet away from the hull! An order had been given to stream FXR, and sure enough the torpedo headed for it and blew up well astern!"
Rocky Schoenrock, Sonarman, USS Inch (DE-146)
(http://www.de220.com/Armament/Decoys/Decoys.htm)
The biggest advantage is that electrics did not leave a visible bubble trail on the surface. That was visible for quite a distance in good visibility. And while merchants didn't have personnel manning hydrophones, they
did have watch crews. If they spotted the bubble trail they could immediately detect not only where the torpedo had come from but, more importantly, where it was
headed. Sometimes even a minute's warning was enough to initiate a course change sufficient to generate a miss. As well, electrics left no 'back-trail' for an escort to follow and trace.