We have a new record-holder.
On march 11, 1943, U-432, a veteran VIIc on its 8th war patrol but w. a green skipper sank a merchantman. Incredibly, instead of taking evasive action, the boat stayed at periscope depth and the officers broke out champagne to celebrate...
...When the corvette
Aconit carried out a standard sweep, it was also surprised to find U-432 still at PD.
Aconit dropped 10 DCs which caused extensive damage to U-432 and drove it down to
1,000 feet (307 meters). This depth "...so terrified all hands.." that the skip decided to surface at which point it was quickly sunk by the escorts. There were 20 survivors.