A mother and her son were found, three hours apart, in the sea off East Coast Park on Monday.
She was saved but, a short distance away, her baby was not as fortunate.
Three hours after Lim Ann Nee was rescued by Police Coast Guard (PCG) officers, her 17-month-old son was found floating in the waters near Bedok Jetty.
It is not known exactly how far apart they were in the sea.
The mother is a 44-year-old Singaporean married to a Frenchman.
The naked body of the baby was spotted by people on a passing boat. The boatman alerted the police.
The baby was taken by police to the PCG headquarters at Pulau Brani, where Singapore Civil Defence Force (SCDF) paramedics pronounced him dead.
Lim was arrested in connection with the infant's death, said a police spokesman on Tuesday. She was charged in court with murder on Tuesday afternoon.
Around 3pm on Monday, the infant's grandfather, Mr Lim Joo Hing, had put up an alert on Facebook, asking for help to locate the baby.
In an interview with Chinese evening newspaper Shin Min Daily News at the mortuary on Tuesday morning, he said his daughter and her son had gone for a walk despite the rain on Monday afternoon.
When they did not return, a frantic Mr Lim decided to get help by asking friends and family members to keep a lookout.
At around 6pm, PCG officers on their patrol craft saw Lim struggling in the sea off East Coast Park.
A police spokesman said she was safely brought to shore and taken to Singapore General Hospital for treatment. She was conscious throughout.
About three hours later, her child was found floating in the sea about 300m from Bedok Jetty. Boat driver Abdul Mutalib, 50, alerted The New Paper and the police.
Mr Lim also told Shin Min that his daughter recently lost her job in a technology firm.
"She got divorced about eight years ago, and got married again about five years ago, to a French national," said Mr Lim. The couple live in Singapore.
Shin Min reported that the boy was seen wearing an orange shirt and red shorts before the incident.
But when he was found by Mr Abdul, he was naked.
Mr Abdul was ferrying four passengers on his 12-passenger boat from Changi Pier to Marina South when one of his passengers saw something floating in the water.
"When we drove the boat closer, we realised it was the naked body of a boy," he said.
"One of my passengers wanted to take the body out of the water, but I refused as the boy was already dead and I didn't want to mess with evidence."
Referring to his grandson, Mr Lim said: "Just thinking about what happened makes me very sad."
If found guilty, Lim faces the death penalty.
Get The New Paper for more stories.