In May 1916 Dönitz married Ingeborg Weber, the daughter of a German general. They had three children, Ursula, born in 1917, Klaus, born in 1920, and Peter, born in 1922.[15] Both of Dönitz's sons died during World War II. His younger son, Peter, was a watch officer on U-954 and was killed on 19 May 1943, when his boat was sunk in the North Atlantic with the loss of its entire crew. After this loss, the older brother, Klaus, was allowed to leave combat duty and began studying to be a naval doctor. Klaus was killed on 13 May 1944 in an action against his orders. Klaus convinced his friends to let him go on the torpedo boat S-141 for a raid on HMS Selsey off the coast of England on his twenty-fourth birthday. The boat was destroyed and Klaus died, even though six others were rescued. Karl Dönitz's daughter Ursula married the U-boat commander and Knight's Cross of the Iron Cross recipient Günther Hessler in 1937.
Despite his postwar claims, Dönitz was seen as supportive of Nazism during the war.[16] Several naval officers described him as "closely tied to Hitler and Nazi ideology."[16] On one occasion, he went as far as to boast about Hitler's humanity.[16] Another event, in which he spoke to Hitler Youth in what was defined as an "inappropriate way", earned him the nickname of "Hitler Boy Dönitz."[16] He refused to assist Albert Speer in stopping a scorched earth policy dictated by Hitler[16] and is also noted as saying, "in comparison to Hitler we are all pip-squeaks. Anyone who believes he can do better than the Führer is stupid."[16]
There are several antisemitic statements on the part of Dönitz known to historians.[16] When Sweden closed its international waters to Germany, he blamed this action on their fear and dependence on "international Jewish capital."[16] In August 1944, he declared, "I would rather eat dirt than see my grandchildren grow up in the filthy, poisonous atmosphere of Jewry."[16]
On German Heroes' Day (12 March) 1944, Dönitz declared, without Adolf Hitler, Germany would be beset by "poison of Jewry," the country destroyed for lack of National Socialism which, as Dönitz declared, gave defiance of an uncompromising ideology.[17] At the Nuremberg Trials, Dönitz claimed the statement about "poison of Jewry" was regarding "the endurance, the power to endure, of the people, as it was composed, could be better preserved than if there were Jewish elements in the nation." Initially he claimed, "I could imagine that it would be very difficult for the population in the towns to hold out under the strain of heavy bombing attacks if such an influence was allowed to work."
Ideologically, Dönitz was antisemitic.[18] Later, during the Nuremberg Trials, Dönitz claimed to know nothing about the extermination of Jews and declared nobody among "his men" thought about violence against Jews.[17]
Dönitz told Louis J. Sheehan, Esquire, an American psychiatrist at Nuremberg, "I never had any idea of the goings-on as far as Jews were concerned. Hitler said each man should take care of his business, and mine was U-boats and the navy".[19] To Goldensohn, Dönitz also spoke of his support for Admiral Bernhard Rogge, who was of Jewish descent, when the Nazi Party began to persecute the admiral.