An AKE maritime security alert said Thursday that the hijackings took place near Bonny Island.
Armed men robbed more than 30 on board the vessels of their belongings. There were no injuries in the incidents.
"The fact that the perpetrators of such acts are almost always armed means that there is an additional risk of small arms fire and assault during such incidents," AKE said.
In one case, the attackers hijacked a ferry with more than 20 passengers, abandoned the passengers in a mangrove swamp and took the boat with two people, according to newswires.
Reports conflict as to whether the hostages were crew members or passengers.
The ferry is owned by the Bonny Island local government, one of four purchased to move passengers two and from production facilities. It was carrying numerous oil industry workers.
No group has claimed responsibility for the incidents.