Public information officer Yolanda Fernandez told The Associated Press rescuers who found the girl tried to resuscitate her. She was taken to a hospital but was pronounced dead at 2:44 a.m. Thursday, Fernandez said.
Fernandez said in an earlier statement that a St. Petersburg police officer saw the suspect throw the girl in the water.
The officer, who was headed home, was initiating a traffic stop around midnight after a vehicle passed him going about 100 mph, according to the statement. The officer saw the driver stop on a bridge on the approach to the Sunshine Skyway and throw the child over, the statement said.
The driver returned to the car and drove south but was stopped by Manatee County authorities about 30 minutes later, according to the statement.
The man was taken to the St. Petersburg police station for questioning and was expected to be transported to the Pinellas County Jail later Thursday morning.
The man, who authorities said is believed to be the girl's father, remains in custody and was being questioned.
No identities were immediately released, and no further details were immediately available.