She returned home a celebrity, and continued to break records, setting a world record for the 400-meter in 1949. She competed in the 1952 Olympics in Helsinki, Finland, but she had had knee surgery to remove cysts and did not win a medal.
Ms. Andersen was a champion but also a realist. She had earned glory, flowers and, once, she said, a bicycle, but there was no money in it. So in 1953 she immigrated to the United States to look for opportunities. Open water swimming, also known as marathon swimming, could be lucrative, with prizes of about $3,000 (about $33,000 today), and she settled in Long Beach to pursue it.
Greta Marie Andersen was born May 1, 1927, in Copenhagen, to Peter and Charlotte Andersen. Her father had been a gymnast who won a silver medal in the 1906 Intercalated Games, a kind of interim Olympics held in Athens.
Greta was 12 when in the spring of 1940 the Nazis occupied Denmark. Her parents feared she might be raped by the German forces, so they chopped off her hair and dressed her as a boy for the five years of the occupation. Afterward, her father encouraged her to learn to swim in a local community pool. A former Olympian, Else Jacobsen, spotted her talent and became her coach.
Her marriage to Mr. Sonnichsen, a high school football coach who had been her trainer, ended in divorce, as did an earlier marriage to Helge Jeppesen, a Danish engineer. She is survived by her husband, Andre Veress, a doctor.
For decades, she ran a swimming school and health spa in Los Alamitos, Calif., teaching swimming to students of all ages.