Simon Lau Sai-man can dive 50 metres on a single breath, but believes he can go much further.
He may be right. The record for free-diving, the extreme sport that has become his passion during two decades of diving, is more than three times that.
But it is the sensation of being deep under water with no special equipment and getting close to nature that motivates him, rather than a desire to test his physical limits.
'Not only do you feel like a fish in the water, but all the other undersea creatures treat you like a fish. At that moment, you are at one with them,' the 36-year-old diving coach said.
This does not happen in conventional diving because the breathing equipment and buoyancy control devices produce clouds of bubbles that frighten the fish away, he said.
'Once you realise the fun of free-diving, you will never leave it. It brings you to such a different world where you will forget all your troubles,' Mr Lau said.
But it is not just a matter of throwing away your air tanks.
Free-diving requires long hours of rigorous training and development of breath-control techniques, and mistakes can be fatal.
'It is probably the most difficult sport in the world, considering you cannot breathe during the whole process,' Mr Lau said.
He has enjoyed water activities since he was a child and bought his first diving gear when he was 16, using his first pay packet from a part-time job.
Later, he chose to work as an insurance agent, partly because it offered him a 'flexible schedule' to continue his hobby.
'But I was often out of reach of my customers when I was out at the sea.'
In 2004, Mr Lau decided to quit his job and became a full-time diving coach while running a diving business of his own. He also worked as the executive director of the Hong Kong Underwater Association from 2004 to 2005.
'At the busiest time, my three diving suits never got dry,' he said.
The diving lover is applying for an international free-driving coaching licence, and plans to devote all his time to the promotion of the sport in the city.
He said the development of free-diving in the city had been slow compared to many other places, with few people participating in it. 'Free-diving is not as dangerous as many people may think. In fact, the human body can endure great depths and a lack of oxygen under water far beyond its normal capacity,' Mr Lau said.
The human body, he said, had several natural adaptations it could make in deep water, like a drop in the heart rate, shrinking of blood vessels, and release of more red blood cells carrying oxygen. All of these functions degenerate if the body rarely uses them.