A 31-year-old man died while diving with his friends in Sai Kung last Saturday evening.
The man, surnamed Chan, took a boat with six friends to Bate Head, where the water was about 20 meters deep.
But as the boat prepared to leave at around midnight, Chan's friends discovered that he had not emerged from the water at the appointed time.
Concerned about Chan's safety, they started looking for him.
Chan was missed for about 30 minutes, and his friends later found his body floating at sea.
A 38-year-old friend, surnamed Chung, filed a report with marine police who rushed to the scene and brought Chan, who was then unconscious, to Wong Shek Pier.
He was sent to Prince of Wales Hospital in Sha Tin, where he was certified dead.
Police said an initial investigation showed no foul play in Chan's death.
But a post-mortem examination will be conducted to confirm the cause of his death.
It is understood that Chan had a history of skin diving and spearfishing.
Bate Head is a challenging diving site with a sandy sea bed off a rocky reef, according to the Hong Kong Underwater Association's guidebook.
The area is deemed unsuitable to beginners, and experienced divers should exercise caution, according to the guidebook.
It also warns that Bate Head is inappropriate for diving and training if wind to the east, southeast, south and southwest is greater than level two on the Beaufort scale.
Oliver Ng Yau-cheong, a veteran diving instructor, said leisure divers rarely dive at Bate Head at night in winter as underwater scenery is dull.
No dive shops operate vessels for training activities as the diving site is far from major piers, he said.
The site is also close to the mainland border, he added.
"I've heard about some divers renting boats and spearfishing at Bate Head at night, despite the cold water and dangerous environment," he said.
He added it was common for spearfishing divers to ignore the buddy system, opting to dive and hunt individually.