Rosie Fan Hiu-yi and two other learners were diving to 3 metres under the guidance of a male instructor from a rented boat about 20 metres off a pier at Wu Lei Kiu. 

The 27-year-old boat operator, surnamed Ma, dived into the water and pulled Fan up at about 3.10pm when he realised she was in trouble.

Fan was unconscious when pulled into the boat. Mr Ma rushed her to Wong Shek Pier and an ambulance took her to Tseung Kwan O Hospital, where she was declared dead at 4.40pm after resuscitation efforts failed.

A police officer said the dive was Fan's first in the sea after practice dives in a swimming pool.

Police said there was nothing suspicious and a postmortem examination would be conducted.

As well as practising at Princess Margaret Hospital, Fan had been seconded as a medical and health officer to the infection control branch of the Department of Health's Centre for Health Protection.

A spokeswoman from the Hospital Authority said last night the family had been contacted to see whether members needed any assistance. She said the department expressed its deepest condolence to them.

Fan was accepted by the University of Hong Kong's Faculty of Medicine in 1999 and graduated in 2005.

In 2001 she had joined a research programme with Medecins Sans Frontieres, in which she visited hospitals in villages of ethic minority groups in Guangxi province.