Police are yet to establish the circumstances under which the incident happened.
The lady's body was retrieved the same night but it had no identification documents.
"We only managed to recover the body of the woman. It's like she had tried to swim out and had moved towards the bank but unfortunately, she didn't make it," diver Martin Ngugi said.
The man's body and the car were retrieved the following day.
His wife later said he had told her the previous day that he was attending a meeting in Juja.
The location where the incident happened is a popular picnic site where revellers like to frequent and chill while engaging in merry-making.
Police said on Thursday autopsy results showed that the lady died from downing and her hand had bruises.
Gov't spokesman issues statement after KDF chopper crashed