It comes amid an earlier search of the water off Victoria Pier in Old Portsmouth.
HM Coastguard has confirmed an incident at 5.30pm near the Hovertravel slipway at Clarence Pier in Southsea.
The Hovercraft vessel Solent Flyer was not involved in the incident, the company confirmed.
A South Central Ambulance Service spokesman said: ‘The incident in Old Portsmouth earlier was in relation to a woman in her 40s who was found unresponsive in the water by the Coastguard and recovered to Camber Dock Lifeboat Jetty where they were met by our team comprising two ambulance crews, a paramedic team leader and an emergency doctor.
‘After initial treatment the patient was then taken to the Queen Alexandra Hospital where she was handed over to colleagues in the emergency department in a life-threatening condition.’
Eyewitnesses report an ambulance arriving near Portsmouth Point – Spice Island – with a Gosport and Fareham Inshore Rescue Service team in attendance and the search and rescue helicopter surveying the scene.
A Ministry of Defence police boat has also been seen assisting the search of the water.
Rayleigh White, who was drinking outside the Still and West pub at the time of the incident, said that a search and rescue helicopter was seen hovering over the area 'for a long time' while police boats scrambled to the waters beyond Spice Island.
She said: 'We got here about 5.30pm and within an hour of us being here, the search started.
'The bar staff said there were police outside. I saw three police boats go past from the harbour. And there was an ambulance outside in the car park.
'It looked like it was a well coordinated rescue. But it was very worrying.'
Emergency services, including Hampshire Constabulary, have been contacted for further information.
The helicopter is not currently on scene, according to flight tracking websites.
More to follow
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