"Emalynne was lying on top of McKenzie and not responding," said the girls' father, Marcus Thomas.
The parents didn't know it at the time, but the girls had been poisoned by carbon monoxide coming from the houseboat's generator. They pulled the children up onto the houseboat and called for help.
"Emalynne was just laying there. I was holding her. Her eyes were rolling back," Britni Thomas said. "It's very hard to describe holding your lifeless child in your arms."
The girls were taken to a hospital. Emergency room doctors called the Georgia Poison Center for guidance on treating the young children.
"The treatment regimen includes oxygen and, in some cases, super oxygen or HBO (hyperbaric oxygen)," Dr. Gaylord Lopez said.
The girls spent the night at a hospital and were given oxygen to dilute the carbon monoxide.
"You never want to see your kids with oxygen being given to them, stuck with a lot of needles and IVs," Marcus Thomas said.
The parents said they are adding another safety precaution to their days at the lake.
"Stay away from the backs of boats, running motors," Britni Thomas said.
Some experts say there could be as many as 250 drownings each year by people who have been overcome by fumes from boat generators or engines.