"The provisional toll is three dead: a woman of 20 and two children whose bodies were found by the police maritime unit," Joel Garrigue, prosecutor on Mayotte told AFP.
"Seven or eight people were brought to land and estimates are that around 30 passengers were on this small boat that capsized," he added.
He said efforts were underway to find the rest of the passengers, but strong tides were making the search difficult.
Such accidents are common in Mayotte, part of the Indian Ocean archipelago of the Comoros that opted to remain under French rule when three other islands chose independence in 1975.
Many Comorans hoping to find work or medical care in Mayotte board rickety fishing boats to attempt the risky voyage, often running aground on coral reefs.
Some two-fifths of Mayotte's 200,000 inhabitants are thought to be undocumented immigrants.
