I'm not sure that there were many sub vs. sub engagements during WWII. The only one I am aware of was a British sub firing at a U-Boat late in the war, and I don't remember it being decisive. I'm not entirely sure what the case was in the Pacific, but there were a lot of PT boats in that theater. I would imagine that attack against subs would likely take place, while above the surface charging batteries. Bombers on patrol would be the most prolific predator.