Dylan Cecil was on holiday with his family when he slipped from the jetty at Burnham-on-Sea, Somerset, at 6pm on Sunday.
His mother, Rachel McCollum, said he fell into a “whirlpool” and she knew she would not see him again despite her desperate efforts to save him.
Underwater rescue organisation SARbot UK was on the scene last night and is using sonar equipment which is more sensitive than that used by police in the search.
The charity’s Twitter feed said it will also use an underwater robot.
Dylan, from Kettering, Northamptonshire, was visiting his grandparents in nearby Brean with his mother, father Darren Cecil and two younger sisters.