The BBC reported the girl's younger bother who had been on the beach saw his sister had fallen and telephoned the coast guard for help. The father had jumped in the water to rescue his daughter.
The family was not immediately identified.
Rescuers found the girl treading water, but her father was found face down in the water.
"Time was certainly of the essence. The little girl was getting weaker and weaker," said Anthony Williams, of the Boscastle Cliff Rescue Team. "We estimate that she had less than a minute left of survival time."
Williams said the water was about 50 degrees Fahrenheit so he had only praise for the little girl. "She's absolutely amazing."
The 42-year-old father was pulled from the water, but died in the hospital. The girl is being treated for hypothermia.