A 59-year-old doctor from Calcutta died on holiday in Thailand moments after complaining of breathlessness while doing the Underwater Sea Walk, a guided marine stroll listed among the must-do activities during a trip to Koh Larn near Pattaya.

Gynaecologist Sukhendu Kumar Kundu, who used to run a nursing home at Behrampore in Murshidabad, had gone for the underwater walk on March 6 with a local tour operator, family members said on Tuesday.

Kundu was brought out of the water because of respiratory distress and taken to a government hospital in Pattaya, where doctors declared him dead. The death certificate issued by the Institute of Forensic Medicine, Police General Hospital, states that Kundu died of 'asphyxiation by drowning'.

'Freak accidents do happen in adventure sports and we always advise tourists to be extra careful and book through authorised agents only, who can provide proper safety guidelines,' said Anil Punjabi, chairman of the Travel Agents' Federation of India (east).

More than 70 per cent of tourists from Calcutta visiting Thailand take the Underwater Sea Walk, he said.

Kundu had reached Bangkok on March 2 as part of a tour conducted by Club 7 Holidays. Wife Srila didn't go because she wasn't fit enough to accompany him on this holiday, sources said.

The Club 7 group reached Pattaya on March 5 and set out for a local sightseeing trip the next morning without Kundu, who apparently couldn't wake up early. 'My father didn't wake up early to join the group for local sightseeing. He went for an underwater walk instead through a local agency,' elder daughter Sushri Basu Kundu said.

Sushri and husband Debarshi Basu were in Hyderabad when Club 7 Holidays contacted them to say there had been an accident. 'They said my father had most probably suffered an accident after he went for the underwater walk. In the evening, they called again and said my father's body has been identified,' Sushri recounted.

'All of us are in a state of shock. He was absolutely fine when he went to Thailand,' she said.

Younger sister Subhashri and husband Debsubhra Chakraborty, who are based in Singapore, rushed to Thailand on March 9 and brought Kundu's body to Calcutta two days ago with the help of the Indian embassy there.

Club 7 officials said Kundu had been 'missing' from his hotel since the morning of March 6.

'He couldn't wake up early and so missed the sightseeing. Our agent in Pattaya informed us that Kundu was not in his hotel,' said Subhra Mukhopadhyay, sales head, eastern India of Club 7. 'Our agent contacted the local police after he didn't return that night. The hotel informed him the next day that a person holding an Indian passport died after the underwater walk at Koh Larn. He was identified and we informed the family members.'

Doctors said people with lung conditions like uncontrolled asthma and chronic obstructive pulmonary disease should avoid any adventure activity underwater.

'If the asthma is controlled and lung function is normal, there is no problem. But people suffering from pneumo thorax or leakage outside the lung should avoid going deep because of the atmospheric pressure underwater,' said Sujeet Rajan, respiratory physician at Bombay Hospital.

He said responding to warning signs like breathlessness, headache, light-headedness and loss of balance underwater was also crucial to survival.

'Gas accumulated in organs like the lung, intestines or sinus are released faster than usual into the blood circulation system, causing such problems. It can cause embolism and can even be fatal.'

A 45-minute boat ride from Pattaya, the island of Koh Larn is said to be ideal for the underwater walk because of its corals and marine life. The boat takes tourists to a ship equipped with everything required for the dive.

Two trained divers accompany each tourist 20 to 25 feet deep, where one is allowed to spend half an hour on an average experiencing the wonders of the marine kingdom. The walk is conducted only in zones known to be free of sharks, a tour operator said.

One diver plays the guide while the other captures videos and comes up before the walk is complete. A helmet with a pipe attached to it from the ship supplies oxygen. There are also smaller helmets attached to small oxygen containers.

Tourists are explained the do and don't before the underwater walk and made to sign a bond. They are told that anyone who feels uneasy or has respiratory distress should raise a thumb to be immediately taken to the boat.

'It's a great experience. There is no rope attached but the divers help you. They give you a packet of bread to attract fishes,' recalled Rajdeep Bhattacharjee, an income-tax official who had visited the island with his family last Durga Puja.

Rajdeep's son Tritoy had difficulty breathing and was brought up immediately after he gestured with his thumb to say he was uncomfortable.