The teenagers were swimming on boogie boards about 100 yards offshore when they noticed a dark shadow in the water, authorities said. The other swimmer was not injured, Walton County Sheriff's spokeswoman Donna Shank said.
Tim Dicus, 54, who had been surfing when he heard a scream from the water, found the girl in the center of a bloody circle of water and said much of her thigh was missing, revealing the bone. The girl's friend had begun swimming toward shore.
"I immediately paddled over and found her floating face down in the center of the blood pool," Dicus told The Associated Press. "And right next to her was the shark, about to come up and attack her again."
Dicus said he put the girl on his surf board and the shark — which he said appeared to be a bull shark about 8 feet long — went after her hand.
"He just followed us right to the beach," Dicus said, adding that he punched the shark on the nose when it tried to attack him. "He was determined to finish lunch. I hate to put it that way, but that was what he was trying to do."
Two other swimmers came with a raft and helped tow the girl to shore.
The girl was taken to a hospital where she was pronounced dead, Shank said. She was on vacation from Gonzales, La., but her name was not immediately released.
The attack happened near the Camping on the Gulf Holiday Travel Park, about 45 miles east of Pensacola. Swimmers were ordered out of the water along 20 miles of crowded beaches shortly after the attack. It was not immediately clear whether they will be allowed back in Sunday.
Robert Goodwin, 12, of St. Louis, Mo., said he was ordered out of the water by authorities. "I didn't know that when I was told to get out it was a shark," he said. "I was like, what? Wow, that's not cool."
Added Goodwin's father, Mark: "It was just an eery feeling to see folks sitting there on the beach."
Florida averaged more than 30 shark attacks a year from 2000 to 2003, but there were only 12 attacks off the state's coast last year, according to statistics compiled by the American Elasmobranch Society and the Florida Museum of Natural History.