I'm reading Michael Gannon's
Operation Drumbeat (which I highly recommend, BTW). He quotes from an "admonition" message (no date given) from BdU concerning the appropriate response of a U-boat to a
surprise attack by aircraft.
"For the boat to dive safely in an aircraft situation, the plane must be at least 8,000 meters distant. If in doubt remain surfaced. During a recent surprise attack on a group of three U-boats, one boat made the mistake of submerging, It was bombed while diving and lost. The surface defense of the others was successful. Moral: When surprised stay above and shoot. Diving is death."