An unidentified man died on Sunday after jumping off MV Harambee ferry at the Likoni crossing.
The vessel had been heading from the Mombasa mainland to the island when the incident took place at about 1.30am, Kenya Ferry Services (KFS) said in a statement on Sunday.
KFS said the man did not seem to be in the right frame of mind.
"The man was dressed in a white top and a black trouser. He left his seat, dashed to the aft prow and dove into the water," KFS said.
A police report obtained by the Nation said the man jumped off midstream the 500-metre channel.
The KFS said rescue efforts were immediate but were yet to yield fruits due to tidal effects and darkness.
It said the search for the man's body was ongoing, the team including maritime police and local divers.
MV Harambee is the ferry from which a woman's car slid into the ocean on September 29.
Kenya's rescue services were questioned after it took 13 days to locate the car and retrieve the bodies of Ms Mariam Kighenda and her daughter Amanda Mutheu.
The recovery involved a team from South Africa.
Following that tragedy, the KFS was blamed for not having an emergency team on standby. To date, it has none.
As part of its safety features, the service says it can only provide fire-fighting equipment and on-board guards.
The parastatal does not have an emergency team of divers or lifeguards and relies on the Kenya Ports Authority (KPA), local divers and the Kenya Navy.
More than 300,000 people and 6,000 vehicles use the channel daily.