The woman was part of a large group who had gone to celebrate the longest day on the island of Egilsay.
Coastguards said she was taking an early morning swim when she got into trouble. Two kayakers went to her assistance, but also got into difficulty. They were all taken safely ashore.
Kirkwall lifeboat and the Shetland Coastguard helicopter were launched at about 6.45am yesterday.
The helicopter landed the trio at Carness, where they were met by ambulance and Kirkwall Coastguard Rescue Team. The woman is being treated in Balfour Hospital in Kirkwall. The kayakers declined treatment.
A Coastguard spokesman said: "The woman was conscious when she was taken to hospital."