An air and sea search for the boy, from Kettering, Northamptonshire, this morning changed to an operation to recover a body, the Coastguards said.
Dylan's parents frantically tried to rescue him after he entered the water on the south side of a slipway at Burnham-on-Sea on Sunday evening, but were unable to reach their son.
The youngster had been on the jetty with his parents, Rachel McCollum and Darren Cecil, and his two sisters.
Police said Dylan had wanted to get closer to the sea when he slipped off the edge and went under the water.
His close family are being comforted as they remain at the scene on Monday morning, anxiously waiting for news.
Superintendent Keith McCoubrey said: "As you can all imagine this has been an extremely difficult time for the family who tried in vain to save him and were pulled from the water and treated by ambulance.
"They have been left traumatised by the whole incident and have asked that you to respect their privacy.
"A police family liaison officer is providing support and keeping them updated on the progress of the rescue attempt."
Four lifeboats, five coastguard rescue teams, police and the RAF helicopter have all been looking for Dylan.
Authorities said the search is being made more difficult by strong tides at Burnham-on-Sea.
On Sunday, a member of the public witnessed the Cecils' desperate attempts to save their son and called the emergency services.
A Swansea Coastguard spokesman earlier confirmed the status of the operation: "The boy will have been in the water for 16 hours now. It is a recovery mission, and the search is continuing."
The Maritime and Coastguard Agency said a search - initially involving a helicopter, Coastguard rescue teams and two lifeboats - began after the alarm was raised.
A Coastguard spokesman said the air and sea search was scaled back at dusk but continued throughout the night, with more resources added at first light.
Police said divers will attend the area to conduct a further search and a cordon remains in place.