The 4.75-metre boat carrying the woman, a man and two children, all German nationals, capsized late Saturday afternoon in the natural harbour, according to a statement from French maritime authorities.
Other boaters rushed to the area and rescued the three other passengers, but found the woman unconscious under the motorboat's hull.
She was taken by helicopter to a hospital in the town of Vannes but was
later pronounced dead. Investigators have opened a probe into the incident.