Chiropractor Monte Campbell said his wife, Diana Campbell, first noticed the baby.
CBS New York says authorities took the child from the water and rushed him to a local hospital and was pronounced dead.
Tourists from Oklahoma discovered a baby lying unconscious and unresponsive near the Manhattan side of the Brooklyn Bridge. "I called 911. At that point, I thought it was a doll", Monte Campbell said.
Police said a pedestrian flagged down two officers in a patrol auto near South and Dover streets around 4 p.m. and told them there was a child in the water.
Diana Campbell posted a news story about the incident on her Facebook, adding, "Asking for prayers for this attractive baby boy. someone out there knows what might have happen to him".
A New York Police Department officer stands guard as authorities investigate the death of a baby boy who was found floating in the water near the Broo
Police say no parents or guardians were found at the scene. There was no respiration nor pulse. The Medical Examiner is working to determine the baby's cause of death.
Police climbed over the safety railing to retrieve the baby, then brought him to the pedestrian walkway where they performed CPR, officials said.
The location near the South Street Seaport on the heavily-trafficked East River, a tidal estuary subject to strong currents, divides Manhattan from Brooklyn.
Unsure if someone else had gone in with the baby, rescue crews continued to search the water Sunday night. Both park-lined shorelines are usually teeming with tourists this time of year.