The mother, who has been named as Paulina, is said to have come from Grudziadz, Poland, and was a single parent to her son, Lech. A murder probe has been launched in Sweden and Poland.
Initial media reports said a child fell into the sea and the woman jumped in after him but video from the ship did not match this version of events, a spokesperson said.
Polish police say a 7-year-old child who fell overboard from a passenger ferry in the Baltic Sea and his mother, who jumped into the water after him, are both dead.