Musa Ahmed was on a day trip to West Mersea, Essex, on 22 July, when he and five relatives got into difficulty in the water.
While he was pulled from the water and airlifted to hospital, Shelina Rahman, 41, and 15-year-old Sameeha, died at the scene.
Musa's family said he died "in a state of ease and peace" at the Royal London Hospital on Tuesday evening.
The group had all travelled to the island, near Colchester, as a family from Ilford in east London that morning.
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