Organisers of Sunday's cross-harbour swim will use sonar technology to identify swimmers who may be in danger of drowning.
The Hong Kong Amateur Swimming Association said yesterday it would use two of the hi-tech detectors, mounted on boats, to spot swimmers who dropped below a certain level in the water.
The splashing of 1,800 competitors in the event could make it difficult to spot swimmers who encountered problems and started to drown, it said.
Ronnie Wong Man-chiu, the association's voluntary secretary, said each sonar detector was effective over a 300-metre radius. One would be placed in the middle of the 1.5 kilometre route and the second near the finish.
This meant the first 30 per cent of the route would not be covered "but we believe the racers should be physically okay at the beginning".
"If anyone starts to drown we will know about it immediately, and the rescue can begin right away," Wong said.
The use of the sonar - borrowed from Shanghai's sporting bureau - will cost the organiser about HK$100,000.
Wong said the sonar was not being used in response to the October 1 sea tragedy off Lamma Island in which 39 people drowned after a ferry collision.
Other safety measures for Sunday's swim include a fleet of 120 boats that will monitor the competition - up from 100 last year. A 190-member rescue and medical team will also be deployed.
Begun in 1906, the race last year was held for the first time in 33 years after it was suspended because of the harbour's pollution.
The race will cover a new course this year, from Lei Yue Mun on the Kowloon side of Victoria Harbour to Sai Wan Ho on Hong Kong Island.
Swimmers were advised to get to the Lei Yue Mun starting line in time for a warm-up of at least 15 minutes, said Fan Wai-tim, a coach with the Hong Kong Regional Swimming Squad.
Fan said the expected water temperature of 23 to 26 degrees Celsius should be "ideal".
A viewing area will be set up near the starting point at the Lei Yue Mun typhoon shelter for friends and families.