Ayman Ksebe was swimming with his older brother in shallow water off Sandringham Beach when he fell into a deep water trough on Sunday just before 4pm.

Police said his brother tried to save him and watched as he disappeared beneath the surface in Botany Bay.

The boy’s mother, Rouba Ksebe, screamed on the beach when she was told her son had gone missing in front of the Georges River 16-foot Sailing Club on Sanoni Avenue.

It is believed the boy did not know how to swim.

He was wearing blue shorts and a blue singlet top.

Onlookers said the current was strong and the beach was unpatrolled when he went missing.

Botany Bay water police coordinator, Senior Sergeant Brett Prentice, said a recovery operation would begin on Monday about 8am. The body was found about 10.30am.

"It’s a matter of finding the poor little thing now," Sergeant Prentice said.

Paramedics helped relatives walk up the beach and tried to console them in the nearby Peter Depena Reserve.

Water police, divers and Marine Rescue NSW volunteers scoured an area of water where Botany Bay meets Georges River until darkness crept in at 8pm on Sunday.

A Westpac Lifesaver Rescue Helicopter, an Ambulance NSW chopper and PolAir also hovered over the water for several hours.

Before the sun set, a fluorescent green dye was dispersed in the water to track which way the current may have taken Ayman.

A number of family members cried as police divers came out of the water, unable to search any more because of the darkness.

Ms Ksebe clutched the hands of paramedics and family as she sat in the sailing club, looking out the window at the unfolding search.

Witnesses say she was unable to move but her gaze never left police as they searched for her son.

At 10pm relatives stood alongside the cordoned off area where little Ayman was last seen.

The search was scaled back during the night but a water police vessel was seen moving up and down the coastline throughout the night.

A police officer was also taken to hospital for treatment.

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