The woman, understood to be in her 30s, went into the water at the Red Funnel terminal at Cowes, Isle of Wight.
It was witnessed by the officer who stripped off and dived into the sea after her. The pair were swept down the harbour by the strong tide but the officer managed to get her to the Island Sailing Club pontoon.
RNLI lifeboat crews were quickly on the scene and provided emergency care. The woman was initially unresponsive and was given oxygen, warmed and then taken to hospital by ambulance.
The incident happened on Saturday evening.
The rescue drama began at around 5pm when the officer spotted the woman, believed to be in her 30s, enter the water at the terminal.
He leapt into the sea and managed to cling on to her as the pair were swept along by a strong tide towards the Island Sailing Club pontoon.
From the pontoon, the pair were helped ashore by crew from the Cowes RNLI Inshore Lifeboat.
Crew member Dr Will King, together with a surgeon colleague from St Mary’s Hospital, attended to the woman who was unresponsive at the time. She was assessed, given oxygen, warmed and taken by stretcher to a waiting ambulance.
The policeman, wrapped in blankets, was taken to the lifeboat station to recover.
More details: www.watersafetymap.co.uk/map/details/198
“looking across East Cowes” by www.flickr.com/photos/ronsaunders47, License: creativecommons.org/licenses/by-sa/2.0