Answer:
Active sonar was a huge advance. U-boats could run almost silently if needed, so hydrophones were useless at times. ASDIC allowed warships to find out exactly where a sub was hiding, and therefore let them use depth charges. It wasn't perfect, but it presented u-boat captains with a challenge: how do you escape from something that can't be escaped? Anti-sonar coatings made detection harder but not enough to escape location after the convoy or fleet got suspicious.