Attack: The woman was in waters that are known to be home to alligators
Nearby canoers and kayakers heard screaming and realised the reptile was attacking the 37-year-old.
The attack happened near Wekiva Island on Saturday afternoon.
One started hitting the alligator with a paddle as it bit into the women's midsection witnesses said.
Her rescuers were able to get the unidentified woman to shore and called 911.
"You just saw some blood and some bone. There wasn't anything else there," said Richard Ward.
Jakob Frick said he was canoeing with friends when they saw the woman swimming in the water.
"We see the jaws just chomp down on her arm, and it starts spinning around, pulls her underwater, goes back up," he said.
"She's just screaming."
A sign at the property warns swimmers that there are gators and snakes in the area.
The victim lost her arm just below her elbow.
Last night the woman remained in Orlando Regional Medical Centre where she underwent surgery.
In June a father battled an alligator attacking his son, while another man in Louisiana was killed by a gator after mocking the fearsome animal - which was later shot dead.
A dog owner from Alabama was distraught when a large gator jumped from the water and ate his beloved pet.
Comment: See also these other recent alligator attacks -
Father, grandfather rescue boy from alligator attack at Lake Charlotte, Texas
Alligator kills man swimming at marina in Orange, Texas