Historicaly , there where some planes shot down by submarine but most of the times they dived when there was a sight of them.
If i,m Right I red about an Herrkaleun shooting an plane down with the Deck Gun
but its a game, If I spotted an plane I mostly try to dive but when its to close I man the flak gun and try to down them and if he's still alive after his first bombing run after that I crash dive to. because after he dropped the first 2 bombs you got time to crash dive to hide urself