The victims, aged 67 and 60, and the child were bathing near a breakwater in Sa Huynh Ward at about 5.45 pm when they were caught by rough seas and strong currents.
The two women were found dead about 35 minutes later, while search efforts for the child were continuing.
The child was wearing a life jacket but was swept away by the rough seas and is believed to have struck the breakwater.
Authorities said rough seas and unusually strong currents had affected Quang Ngai's coastline over the past four days, leading to several water rescue incidents.
On Saturday, a father and his two children from Hanoi were rescued after being swept out to sea by a rip current at My A Beach in the province.