Should I be keel-hauled?

[UKönig]

Ships and aircraft had radar in 1940. Not as many sets to go around, to be sure, and it was kinda primitive still, but it worked.
The water disturbed by a crash diving U-boat could remain churned up for a good 5 minutes after the fact. So, 10 minutes is probably safe margin. But aircraft have radio, and can report your last known position. Since a submerged boat can't go very fast (and the Tommies know this), you aren't likely to have gotten very far from your dive point, by the time that more planes show up.

You may also be getting gamed by the game. And it's strictly a programming thing designed to make the player more nervous, and less casual about incoming aircraft.