“The boat capsized after hitting a rock … around 6:00 pm (1700 GMT) yesterday,” Ibrahim Farinloye of the National Emergency Management Agency told AFP.
“Eight people lost their lives while we were able to rescue 12 others,” he said.
He said the small, privately-owned passenger ferry had launched from the Ebute-Ero neighbourhood in southern Lagos and was headed north across the city’s vast lagoon towards Ikorodu when the accident occurred.
He said the boat was carrying 20 passengers, all of whom have now been accounted for.
Three weeks ago, an overloaded wooden boat capsized in the Festac Town area of the city, killing 13 while six others had to be rescued. The boat had capacity for only 12 passengers.
Overloading on such vessels is common, as operators often seek to maximise their profits while downplaying safety concerns.
Lagos officials have been trying to promote ferry travel to help ease traffic in the congested city of more than 20 million people, the largest in sub-Saharan Africa.
But commuters have been reluctant to embrace water travel, partly because of safety fears.