In a statement, Sussex Police said the family had travelled down from their home in London to visit Shoreham that day.
The force said the family had entered the water from the beach at Shoreham Fort about 14:00 before getting into trouble.
A significant emergency service response followed including HM Coastguard, the RNLI and South East Coast Ambulance Service and Sussex Police.
“The investigation into the incident remains open, but following review of CCTV footage and initial witness accounts, the current hypothesis is that this was a tragic accident, with no third-party involvement,” a police spokesperson said.
They added that a file will be prepared for the coroner ahead of an inquest.
Ch Supt James Collis said: “This heartbreaking update is one we all feared in the aftermath of Tuesday’s tragic events.
“The impact of this tragedy has been felt across the country, and I know the Shoreham community, emergency responders and all involved will continue to be affected for a long time to come.
“I would like to thank all of the public who have shared their sympathies and kind words.”
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