A baby boy was found floating in the water off of the South Street Seaport on Sunday afternoon and was later pronounced dead, police said.

Passers-by noticed the child — clad only in a diaper and estimated to be about 8 months old — in the water near South and Dover streets in lower Manhattan and called 911 around 4 p.m., according to police and witnesses.

Oklahoma firefighter Monte Campbell, in town on vacation, went into the water to get the child and put him on a nearby embankment.

“My husband knows how to swim pretty well. I told him there was a baby in the water, and we called 911. My husband jumped in there and got the baby out,” said wife Diana Campbell, 41, whose family said she first spotted the baby.

“I couldn’t believe that that was a baby in the water. I’m still in disbelief,’’ said Diana, who had just been dropped off by a ferry at a nearby pier after taking a boat trip to the Statue of Liberty.

Monte performed CPR on the boy but felt no pulse, according to his son Anthony, 19.

“I’m from Oklahoma, a small town of 60,000 people, there’s like no crime . . . it’s just unreal,” the teen told The Post. “You don’t come to New York City and expect to see things like this, see babies floating in the water. I don’t feel good about it.”

A responding NYPD officer also attempted CPR. The boy was taken to New York-Presbyterian Hospital, where he was pronounced dead, officials said.

A police source said the boy wasn’t in the water for very long.

There were no signs of trauma, and the baby’s parents have yet to be found, police said.