* GoddamnedBats: Aircraft, especially ''Allied'' aircraft. Sure they start off as a joke, but by 1944 they can spot you beyond visual range with radar, appear out of nowhere and stick a bomb right on your conning tower on the first pass. The only thing you can do is hit 'crash dive' and pray. And they're ''everywhere''.
* HellIsThatNoise:
** Sonar pings and propeller noises when you are getting chased by escorts, especially after a successful attack (as you might lose all that hard work).
** "[[NightmareFuel Depth charges in the water, sir!]]"