Police say the white car slid back from Mv Harambee mid-stream.
The car was among the last ones to get into the vessel from the mainland side of the channel.
An eye witness Mr Mlei Majaliwa said he saw a woman and a boy in the vehicle.
“I saw the woman and a boy inside the car. She had started screaming and calling for help when the car started to reverse. I threw a life saver at them,” said Mr Majaliwa who was in the same ferry.
Likoni Sub County Police Commander Benjamin Rotich said it is believed that the vehicle had four occupants.
“The woman was the driver, the information we have indicates they were four,” said Mr Rotich.
Eyewitnesses said earlier there was an altercation between the woman and the Kenya Ferry Service (KFS) officials at the mainland side ramp.
“The officials had stopped the woman after she was seen accessing the ferry from the side which is used by people with KFS special passes,” said an eyewitness.
KFS says the cause of the accident is still under investigations as search and rescue efforts are under way.
Neither the car nor the occupants have been retrieved.