I'm afraid you would have to provide a lot more information if you want an opinion as to its worth. Photos would help too as well as a detailed description of its condition.

There were many clocks used aboard a U-boot. If it's in a wooden box on a moving gimbal it's probably a chronometer which would be far more valuable than say an ordinary wall clock or observation watch. What colour is the dial (the face of the clock)? Black or silver? Is there a manufacturer's name on it such as Wempe, Junghans or Kieninger?

Depending on the amount of detail you can provide I'm sure you there would be many interested buyers here (myself included).