Gunboat+Torpedo=DOES NOT COMPUTE!!!!
All joking aside, I don't think U-Boat AA gets terribly effective unless you have two twin 20mm and a twin 37mm. The U-Boat's default AA config (two single slow-firing 20mm and a SINGLE-SHOT 37mm) IMHO is less effective against planes than a single 20mm Oerlikon on the back of a U.S. fleet boat, but that's my experience. I'm far more willing to fight it out with Japanese planes than with Allied planes, even though I love the IXD2!