ASW aircraft that behave more like the real thing, for ex., dropping DCs in your path instead of bombs. Also flying in ways appropriate to the aircraft type, for ex., heavy bombers won't dive-bomb but might fly low level on the attack run. And if they have the range & payload (esp. Sunderlands) they should stick around longer & drop multiple times.

Also coordinated search efforts, i.e., being detected by aircraft, naval or merchant vessels means other units will follow up to stay in contact, with potentially dire results.