The incident occurred last evening at the Girgoan Chowpatty when several idols were being immersed into the sea, a police official, who was at the spot said.
An emergency call was made to fire brigade, he said, adding, with their help the boy was rescued.
He was later taken to GT Hospital in South Mumbai and his identity is yet be ascertained, the officer said.
Yesterday, lakhs of devotees thronged Girgoan, Dadar and Juhu Chowpatty and other immersion sites to bid adieu to Lord Ganesha.