Essex Police confirmed Haider Shamas, from Luton, died in hospital on Saturday, two days after the incident in Clacton that also claimed the life of his 14-year-old sister Malika.
A 15-year-old girl, a relative, who was also got into difficulty while swimming, is expected to make a full recovery.
Witnesses described “distressing” scenes as lifeguards and paramedics scrambled to help the three teenagers.
The trio were spotted struggling near Marine Parade at about 1.40pm on Thursday.
The tragedy has sparked renewed focus about water safety for beachgoers in Britain.
“It was so distressing, ”
Witness Becky Bryant, 40, said: “I saw two people being pulled from the sea and lifeguards trying to do CPR until the paramedics turned up.
“Everyone tried to take turns doing the CPR while the lifeboat went out again and retrieved the third person who seemed not to be in such a bad way.”
The carer from Wivenhoe added: “It was so distressing, everyone was trying to do what they could; lifeguards, air ambulance, they truly worked as a team.
“[The victims] were in a large group of around 30. A lady was crying and screaming as two people comforted her.”
Another witness Karen Jones, 40, said: ““As a parent I would like to see more signage warning of dangers.
“And perhaps schools could reinforce the warning by educating young people.”
She added: “I realised something was definitely wrong when a police transit van drove right along the [promenade] towards the pier.
“I saw a huddle of people on the beach a little way from the pier.”
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