The boy was playing in the water while his family of five, vacationing from Nebraska, relaxed on the edge of the lagoon, when he was seized by the animal, believed to be between 4 feet and 7 feet (1.2 meters and 2 meters) long, Orange County Sheriff Jerry Demings told reporters.
"The father entered the water and tried to grab the child and was not successful," Demings told a news conference just after midnight.
The mother also tried to rescue the boy and a nearby lifeguard was told of the attack, Demings said.