The answer is U-923 commanded by OLt.z.S Heino Frömmer-Carus (also known as Heinz Frömmer). He took his wife on board for a cruise in Kiel Bay and the boat was subsequently attacked and sunk by aircraft on 9th February 1945, all hands lost. His wife's name was Margrit. It was her 21st birthday the day before the incident. The captain was 24.
My source... I had read about the incident recently, but can't remember where. Fortunately, there is some information on ubootwaffe.net: U-923 Crew list
Given the stage of the war when this happened and the risk of taking civilians on board a U-boat, was Frömmer perhaps trying to smuggle his wife out of the country?
The tragic details that would explain why his wife was on board!
topic
Just got a e-mail from Horst with the list of U-923. sunk 08.02.1945 in kiel bay.
When they raised U-923 in Jan 1953 they found the body of Margrit Frömmer-Carus wife of commander OLt.z.S. Heino Frömmer-Carus.
As she was born 08.02.1924, maybe he gave her a 21st birthday present.the trip of a life time.
Sad to say it was not a nice present.