The boat capsized after hitting a rock last evening, 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.
One of the two pregnant women who died was due to get married in 10 days' time. Her fiance also lost his life, said Farinloye.
The incident was the second in three weeks in the waters in and around Nigeria's biggest city, which is surrounded by lagoons and the Atlantic Ocean.
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Twenty passengers were on board.
Last month, 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 megacity of more than 20 million people, the largest in sub-Saharan Africa.