Campbell said he "walked out mid-thigh level" in the East River and retrieved the baby and started CPR.
The body was spotted in the water of the East River under the Brooklyn Bridge by a family who had travelled from Oklahoma to NY for a holiday.
A family visiting from Oklahoma spotted the baby wearing only diapers in the shallow water near the Manhattan shoreline on Sunday at about 4pm.
Tourists from Oklahoma, the Campbell family was taking in the view under the Manhattan side of the Brooklyn Bridge Sunday afternoon when mother Diana Campbell made a heartbreaking discovery.
"It just doesn't seem real.it's just, I dunno, I didn't expect this when I came here", Campbell told New York ABC station WABC.More news: California wildfires: Death toll to 7 as rapid spread continues
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A bystander plucked the baby - who was about 8 months old - from the water, but he could not be saved. They say the baby was suffering no obvious signs of trauma when he was first spotted.
The New York City Fire Department reportedly took the child to the hospital where the baby died.
The East River running between Manhattan and Brooklyn is a heavily trafficked tidal estuary subject to strong currents.
The medical examiner will work to determine the boy's exact cause of death. Both park-lined shorelines are usually teeming with tourists this time of year.
It wasn't clear how long the baby had been in the water.