His Majesty’s Coastguard said it responded on Friday to a report of two children, a boy and a girl, in difficulty in the water near the Royal Terrace Pier in Gravesend, Kent.
The alarm was raised at about 1.35pm.
A boy has since been rescued.
Kent Police’s marine unit and the Coastguard continued a search of the water and a river bank on Saturday.
HM Coastguard said Metropolitan Police boats joined the search operation on Friday, alongside the RNLI lifeboat from Gravesend, Kent Police, and a police helicopter.
The South East Coast Ambulance Service was also sent.
The ambulance service said a “male patient was assessed and treated” at the scene. He was taken to a local hospital for further medical treatment.
