Divers spent several hours talking to her through the hull of the vessel as it was towed in the dark to calmer waters.
The rescue was carried out under the floodlights of Royal Navy helicopters in Alum Bay on the Isle of Wight. A diver went under the hull with an oxygen mask and cylinder to reach the woman through a hatchway. She was then lead underwater to safety.
The woman, who was suffering from hypothermia, and two other members of the trimaran crew were airlifted to hospital in Gosport.
Coastguards were alerted at about 4pm by a flare in the Needles Channel. Two people were found standing on the upturned hull of the trimaran, which was tip over by a freak wave.
A helicopter returning to Portland from another rescue was diverted to the scene. It had already airlifted 10 people from a burning ferry off Guernsey. The 19-strong crew stayed on the Norman Commodore which was able to continue its journey to Guernsey from Portsmouth.