A 36-year-old Singaporean died on Indonesia's Bintan island last Saturday afternoon while doing an advanced diving course.
Mr Yeo Boon Han, an accountant who is believed to be single, had signed up alone for the three-day, two-night advanced open water diving course.
The incident happened during his second dive last Saturday, on the second day of the course.
There were four other dive students in the group.
A spokesman for GS-Diving, which conducted the diving course, told The Straits Times that nothing was amiss during the first dive earlier that day.
She said Mr Yeo did not show any signs of discomfort or illness before the start of the second dive, which took place in waters near Berakit, at the northern tip of Bintan.
But during the second dive, Mr Yeo indicated to his dive buddy that he wanted to ascend to the surface, said the GS-Diving spokesman.
Upon reaching the surface with his buddy, Mr Yeo was said to be unresponsive and was towed by his buddy back to the dive boat.
The dive crew, which comprised an instructor, three divemasters and two divemaster trainees, administered first aid by putting him in a recovery position, followed by cardiopulmonary resuscitation, said the spokesman.
Mr Yeo was also given oxygen.
He was subsequently transferred to a fishing boat and taken to a clinic nearest to the jetty, where the doctor pronounced him dead.
An ambulance then took him to hospital.
The GS-Diving spokesman said that according to the information on the equipment of the dive buddy - a certified rescue diver - they were at a depth of 25m when Mr Yeo gave the signal to ascend.
Advanced open-water diving courses allow divers to reach a maximum depth of 30m.
The Straits Times understands that Mr Yeo's family left for Indonesia following the incident.
The Ministry of Foreign Affairs said in a statement yesterday that it is aware of the incident and that it extends "deepest condolences to the bereaved family for their loss, following the unfortunate diving incident near Bintan".
"The Singapore consulate-general in Batam has met the family to render consular assistance," it added.