According to Sussex Police, the family had travelled down from their home in London on Tuesday to visit Shoreham, which is about 6.3 miles (10.1km) from Brighton.
The force said the family had entered the water from the beach at Shoreham Fort at about 14:00 BST before getting into trouble.
A Sussex Police spokesperson said the investigation remained open, and the “current hypothesis” was that it was “a tragic accident, with no third-party involvement”.
Addressing Saja’s death on Friday evening, 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.”
The family lived in London and were British citizens of Palestinian origin, with roots in the Rashidieh refugee camp in Lebanon.
Their extended family had lived in the UK for many years, the Palestinian Forum said.
Prime Minister Andy Burnham called the incident a “terrible family tragedy”.
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