Five people are feared to have drowned after a boat capsized in Masinga Dam, Machakos County, on Thursday.
Seven others were rescued and taken to Ekalakala Dispensary.
The 12 people were being ferried from Muthesya to Ekalakala market, Masinga Deputy County Commissioner John Odengo said.
"The people were not wearing life jackets and the boat was overloaded," he said.
Mr Odengo said there were bags of maize and motorcycles in the boat.
"Residents use boats to ferry their wares to Ekalakala as today (Thursday) is a market day,” he said.
He said divers had launched a search for the missing five people.