The girl, whom police refused to identify Sunday, her parents and five siblings were on a day trip to the island and had walked along the beach just north of the village area, said Capt. Jay Wiggins, director of Glynn County's Emergency Management Agency.
The girl and her mother were in the water at Seventh Street a short distance from St. Simons Elementary School when she was lost, Wiggins said.
"A big wave hit her and took her under. That's the last the mother saw of her,'' he said.
The beach near the island's south end has a lot of sandbars and drop-offs, Wiggins said.
"People who live around here know this isn't a place to go in the water,'' he said.
Early reports said she had been swimming 100 feet off the beach, but Wiggins said neither the girl nor her mother could swim.
Crews on Georgia Department of Natural Resources boats and on a U.S. Coast Guard boat and helicopter searched into Saturday night.
The search was hampered again Sunday by an east wind that kept the tide from receding and the very rough water, Wiggins said.
At 3:15 p.m., about two dozen volunteers began combing the short stretches of exposed beach and rocks searching for the girl, and an EMA Search and Recovery boat traveled up and down the beach.
"We're probably in a recovery mode. I know it's bad to say that,'' Wiggins told the volunteers.
He also asked them to look carefully.
"She may have been pushed up into the rocks or may be under the sand,'' he said.
By early evening only a shrimp boat among the small fleet of volunteer search craft was able to navigate the rough waters.