There are so many variables to your question that it can't be answered by one all encompassing statement. The tactics used to combat an air attack changed during the war, but were also dependent on other conditions such as: whether or not the u-boot is in a calm or rough sea, the colour of paint the u-boot had, whether or not the u-boot was travelling alone or with another u-boot or even the type of water it was in.

You can find a lot of information on this subject in the U-boot Commander's Handbook which addresses many of the variables I listed above.