The inflatable boat capsized on Thursday in the Rhine River that marks the border between France and Germany, just off the French town of Gerstheim and close to a hydro-electric power station and dam.
French and German rescuers were employing divers and using helicopters in a bid to find the missing girl.
Officials said the four people – part of a group of seven Germans and Romanians who had come from Germany – had taken to the water in defiance of clearly posted warnings against swimming and boating.
A man, 29, and a girl aged 6 both died when the boat capsized, while a 22-year-old man from the same group also lost his life when he jumped in to rescue them.
Two other witnesses of the accident who dived in a bid to save the victims managed to swim to safety.