The girl, believed to be 4 years old, was rushed to St. Joseph's Hospital in Tampa, where she died shortly after arriving, Police Chief Brian Dugan said during a press conference at the scene.
Police believe the suspect drove to the scene of the girl's death and had been focused on a gray Nissan parked in the area with a pair of small shoes underneath, as reported by the Tampa Bay Times.
Police say they found Denson walking nearby and took her into custody.
"Unfortunately at this point there are more questions than there are answers", he said. "We're still digging into Denson and what her state of mind is", said Dugan.
Dugan said Shakayla Denson, 26, stole a vehicle from Jordan Auto Repair on N. 40th Street, a short distance from her apartment, just after 3 p.m. Thursday.
Some witnesses said they could see the girl's hands above the water - then they disappeared.
Denson has previous arrests for driving with a suspended license and theft charges dating back two years ago.
A dive team went into the water at 4:30 p.m. and located an "unconscious child submerged about 75 feet off shore", police said. "Then everybody going full attack mode to make sure she got in the ambulance and then taken to St. Joe's".
The Sheriff's Office released a statement Friday afternoon saying that on June 19, their Child Protection Investigation Division looked into a report of "inadequate supervision" against Denson.
Je'Hyrah Daniels, 4, was diagnosed with autism. "I don't know the circumstances, I don't know what's happening in their lives, but it's killing me".
Police say the girl was approximately 4 years old.