The 13-year-old boy had been in around 9ft of water at Beacon Beach, north of San Diego, at 7am local time (5pm BST) when he was attacked.
The child was taken aboard a kayak by three people who had also been diving for lobsters nearby, with two of the helpers being an off-duty life hard and an off-duty police officer.
Chad Hammel, who witnessed the boy screaming, said he heard the teenager shouting: “I got bit!”
Mr Hammel told NBC 7: «I paddled to him and there was a big wake of blood behind him.
«His entire back was open.
«The shark hit him in the clavicle.
«The shark’s top teeth got him in his cheek.»
Mr Hammel and his companions dragged the boy, who had not been named, to the shore where together they applied pressure to the wounds while holiday-makers and residents called for an ambulance.
The boy was immediately flown to hospital with serious injuries to his upper body.
Lifeguard Captain Larry Giles said despite suffering serious injuries and being in a critical condition, the child is expected to survive the horrific ordeal.
He said: “He was conscious, he was above water, and the bystanders were able to get him up on the beach and he was talking all the way to when he was being transported.”
Mr Giles then praised the three people in the kayak for providing a «tremendous amount of aid to that young man».
Witnesses of the shark attack claimed the beast was 3.3m (11ft) long, but the type of special is yet to be confirmed.
The shark has not been seen since the attack on the boy.
The area on the beach where the incident happened has been closed for 48 hours and a surfing competition was canceled as a result.
in 2017, there were a total of nine reported shark attacks on the US west coast.
Of those, eight were in California and one happened in Washington, according to a report published in the Los Angeles Times.