Three people and a woman they were attempting to rescue from the sea are recovering in hospital.
The woman was pulled unconscious from the water after getting into difficulties at Camber Sands, near Rye, East Sussex, just before 5pm on Saturday.
It appears that during the struggle to help the woman, the three people who were with her waded in but ingested sea water themselves.
The woman was treated on the beach and then airlifted by air ambulance to hospital in Brighton, the Maritime and Coastguard Agency (MCA) said.
The three other people, whose ages and sex have not been disclosed, were taken by road ambulance to hospital but their conditions were not thought to be serious.
An MCA spokeswoman said: "At 4.41pm, Dover Coastguard was called by a member of the public to say that a lady appeared to be in difficulty.
"She had been pulled unconscious from the water which was thigh-deep at Camber Sands. There was a doctor on the beach.
"She was treated on the beach and taken by Kent Air Ambulance at 5.37pm to hospital in Brighton. Three other people who were with her were taken to hospital by road ambulance."
The sea state was described as good and there were no warning flags in place at the time of incident, the spokeswoman added.