Here's something interesting I found about him. Part of the story appears to be true. There's no mention of Dönitz rescuing him. The following is from "Neither Sharks Nor Wolves" citing "Voice of the Third Reich" by Johannes Steinoff, Peter Pechel. Dennis Showalter.

"Another living example of the contradiction between doctrine and practice was Helmut Schmoeckel, who eagerly joined the navy as an officer candidate in 1936 before discovering that his maternal grandfather was “100 percent Jewish” (Volljude), leading to his discharge from the service as a “Mischling” (half-Jew). Schmoeckel’s father nevertheless petitioned for his reinstatement, which was granted in July 1939. After serving aboard the cruiser Admiral Hipper, Schmoeckel entered the U-boat branch in 1942 and in September 1943 as a Kapitänleutnant received command of U-802. He survived the war’s most difficult period and even sank an Allied merchantman on March 1944, and at war’s end he was en route to operations off the New York coast."
-- Mulligan, Timothy. Neither Sharks Nor Wolves (p. 229). Naval Institute Press. Kindle Edition.