Eleven swimmers have been rescued after getting into difficulty in strong currents at Porth Beach in Newquay.
The RNLI said one person became submerged, unconscious and unresponsive during the incident near the mouth of the river on Sunday evening.
Lifeguards were alerted after a large group moved outside the red and yellow flags and into an area of stronger currents and deeper water.
The charity said conditions were challenging because of strong currents, a large swell, spring tides and increased river flow following recent rainfall.
Swimmer found unresponsive
RNLI lifeguard Louis Harris entered the water and reached the nearest group of swimmers, using his rescue board to provide support before bringing eight people to safety.
Lifeguard Elodie Sykes also entered the water with a rescue tube to assist with the operation.

RNLI Lifeguard Elodie Sykes
Meanwhile, lifeguard Harry Barea reached a second group further offshore, where one person had become submerged and was unconscious and unresponsive in the water.
He provided immediate assistance and brought the casualty and two other swimmers back to shore.
The RNLI said the casualty became unconscious again once on the beach and required emergency care, with HM Coastguard and the ambulance service called to assist.
All 11 people were safely recovered from the water, with one handed into the care of paramedics for further assessment.
A member of the public on a foil surfboard also helped during the rescue by assisting one person back to shore.

RNLI Lifeguard Harry Barea
'Could have ended very differently'
RNLI Senior Lifeguard Dave Kelly described it as a "significant and complex incident".
"The teamwork shown by the lifeguards was outstanding," he said.
"With strong currents, deeper water and a large number of people involved, this incident could have ended very differently.
"The quick actions, professionalism and training of the lifeguard team played a vital role in achieving a positive outcome."
The RNLI is reminding people to check local safety advice before entering the sea and to swim between the red and yellow flags on lifeguarded beaches.
Anyone who sees somebody in difficulty in the water should call 999 and ask for the Coastguard.
Daily RNLI lifeguard patrols at Porth Beach are due to continue between 10:00 and 18:00 until 27 September, while Fistral Beach will be patrolled daily until 1 November.
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