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.