PC Nathan Lucy swam out after the woman, who'd jumped into the water from the Red Jet terminal.
Divers Warned Over The Bends
Divers in Dorset are being warned to be careful when surfacing, after four were treated for the bends inside an hour.
The first was at West Bay near Bridport on Sunday 18 August - and the others off the coast of Monmouthshire later that day.
They were all admitted to a special recompression chamber in Poole.
The Coastguard said the first incident occurred just after 1pm when Portland Maritime Rescue Coordination Centre was contacted by a dive boat to say that one of their divers had made a rapid ascent.
The Weymouth-based RNLI lifeboat was sent to the scene and the diver was transferred from the lifeboat to the Coastguard rescue helicopter for transfer to Poole.
The helicopter was met by an ambulance, dive doctor and Poole Coastguard rescue team and taken to the recompression chamber.
Just 15 minutes later the maritime rescue coordination centre was contacted by the Aeronautical Rescue Coordination Centre to say the Coastguard helicopter from Portland was being sent to Chepstow, Monmouthshire, to fly three divers who were showing symptoms of decompression sickness back to Poole.
The three had been diving in a quarry.
The divers were met at Poole helicopter landing site by an ambulance, dive doctor and Poole Coastguard rescue team and also taken to the recompression chamber to join the first diver.
Maddy Davey, Portland Coastguard watch manager, said:
"The most common diving incident we had in 2012 was decompression illness with 40 recorded incidents last year.
"Divers should make sure they are adequately qualified and experienced for the dive they plan to undertake, keeping a close eye on weather and sea conditions, and making their own personal fitness a top priority for safe diving.
"You should also familiarise yourself with new or different gear before planning deep dives and to always dive within your limits.''
Dad-of-two Nick Medlin from Ventnor died after being assaulted outside the Rose Inn pub in the town.
A section of Parkstone Road will be affected between 9.30am and 4.30pm for three days.
A man's died in hospital after a crash in Eastleigh.
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