Looking for a name
 

...of a pub that submariners gathered to party, back in the year 1943 in St. Nazaire. Can anyone know of one? I'm not having much luck finding one.

Das Boot was based on fact and the 2 Hotels (in St-Nazaire France) mentioned are Hotel Majestic & Bar Royal, the latter more of a whore house, & yes plenty of fun & trouble there!:D
But Das Boot is still a novel so I couldn't say they really existed!:)

Thanks Reece. Now if a U-boot crew wanted to throw a surprise birthday party for their captain, what would the banner say in German to mean "Happy birthday Captain" and it would probably be held in the Hotel Majestic, right?

Let's assume, the Kaleun happens to become 35.
Then such banner would probably be inscripted with Alles Gute zum 35. unserem Kaleun. Or Herzlichen Glückwunsch zum 35. Geburtstag, Herr Kaleun. Or just Herzlichen Glückwunsch or Alles Gute zum Geburtstag. If you don't know how to type vowel mutations, ä,ö,ü can also be substituted with ae,oe,ue :).
And yes, they would probably choose the Majestic, as they would deem a smutty joint like the Royale unworthy of their commander. But a party for an NCO would probably be given there... :)

The crew doesn't know exactly how old their Kaleun is, so then if I used Herzlichen Glückwunsch , what exactly does that say in English?

congratulations!

Thanks