Singaporean diver Rinta Paul Mukkam remained missing as night fell yesterday after a second day of searching, with the Indonesian authorities pledging to continue the effort this morning.
Ms Rinta, 40, had been diving off the famed Komodo Island on Thursday when she failed to resurface with her group.
Search efforts have so far spanned a radius of more than 200km.
Ms Rinta had started her dive at about 11.40am that day at a site off Gili Lawa Laut island.
According to one woman in the group of 16 Singaporeans she was in, a boat had dropped them off at the spot and left to pick up other divers.
But Ms Rinta, who is an associate regional sales manager, did not surface with the other divers, said the Singaporean woman, who declined to be named.
She added that it was sunny at the time and they had been briefed by the dive master to be prepared for a drift dive - where divers are carried by the current.
The divers were then split into smaller groups for the dive. It is unclear what happened to Ms Rinta's "buddy". Divers typically dive in groups of two or three as a safety precaution, in a buddy system where members look out for one another.
According to West Manggarai regency police chief Supiyanto, Ms Rinta was in a group with a local diver known as Ajidan.
As they dived to the 4m depth, Ms Rinta stopped and swam back to a shallower 2m depth, while her three other friends continued to dive deeper to 15m as planned.
"When Rinta was diving towards the 2m depth, she signalled to the (local) diver to continue and accompany her three friends to the seabed (at a depth of 15m)," Mr Supiyanto said.
The group discovered that she was missing only when they had completed the dive and were boarding the boat.
"The missing tourist is suspected to have been swept away by currents at the depth of 2m, at the point where she was to surface and get onto the boat," added Mr Supiyanto.
According to those familiar with the area, Gili Lawa Laut, a popular diving spot, is also known for strong currents and relatively cold waters.
Mr Gede Ardana, head of rescue agency Badan SAR Kupang (Basarnas), told Indonesian media that his agency received a missing persons report at 2.30pm on Thursday. His agency then sent a team of six rescuers to the location by motorboat to carry out a search.
Yesterday evening, Basarnas told The Straits Times that boat operators and Komodo-based dive centres were also helping out in the search operation.
The current search-and-rescue efforts are focused on the region west of Flores Island, which is located in West Manggarai in Indonesia.
Ms Rinta's brother Roy Paul Mukkam, a 36-year-old lawyer, posted on Facebook on Thursday evening asking for help and information in locating his sister.
He told The Straits Times that his sister was an experienced diver and had gone on a number of diving trips previously. She had left Singapore on Wednesday and was due to return home on Monday.
Mr Supiyanto noted that there had been similar incidents at the site where Ms Rinta went missing.
"There is such a case nearly every year, and the majority involve foreigners," he said.
According to the Divezone website, the currents at Komodo Island dive sites are very strong, and the sites are reserved for experienced divers.
The website stated: "The current can sometimes reach 8 knots in some places... This current is due to the Komodo and Rinca islands acting like a bottleneck between the Indian Ocean and the Pacific Ocean."
In April last year, Singaporean Vera Neo Qiu Ping, 27, went for a five-day diving trip at Komodo Island and failed to resurface from a dive. Her body was found a day later, 12m underwater.