"Aircraft Spotted!"= C key.
But if for some reason I was unable to dive and had to fight it out, a dual 20mm(probably a quad when it became available) and a 37mm AA gun would probably be my standard fitting.
I'm in mid to late '43 with a couple of careers(nearing 1/44 in one of them) and aircraft are becoming more than a nuisance so I may have to look deeper into what AA guns to mount.
I don't know if the game models slower diving times with all those extra guns topside. If so, I guess you have to balance the pros and cons.