One thing that helicopter pilot Galih Prasetyo Kusno Adi learnt in almost 40 search-and-rescue missions is to keep his spirits up even when an operation extends into days or is hampered by bad weather.

The 30-year-old Indonesian pilot is involved in the search for three divers in Indonesian waters, which entered its fifth day on Thursday (Nov 7).

A Singaporean diver identified as Wang Bing Yang, and two Chinese men, Qin Xue Tao and Tian Yu, were reported missing on Sunday in the waters off Banten province's Sangiang Island in the Sunda Strait. Banten on Java island is located to the west of capital city Jakarta.

As Galih and his fellow rescuers set off on Thursday morning, they faced unfavourable conditions, with high waves and strong winds hindering the search.

Galih told The Straits Times in a telephone interview during his break that Indonesia's search-and-rescue agency (Basarnas) will continue operations till the seventh day, before deciding on a future course of action.

If the rescue team find any clues in the waters, such as personal belongings or diving equipment, the search may be extended, he said.

"The divers' families attend our evaluation meetings every night before returning to their hotel. They are naturally worried and afraid about their loved ones. We have to try our best and stay optimistic about finding the victims," he added.

At least 12 vessels, including navy ships and police patrol boats, as well as two helicopters have been deployed in the search that covers a total of 1,300 square nautical miles. A diving unit has also been activated to comb a designated zone within the search area.

Working from 6 am to 5 pm each day, Galih and other pilots search an area of about 922 square nautical miles. The area covered by air operations is the biggest of the four regions covered by the entire search team.

Basarnas' Banten branch began the search operation after a diving mate of the missing trio alerted the agency on Sunday.

A total of six foreign tourists went out diving in two groups, but the missing trio did not return to base.

A team was quickly mobilised, combing the area in the waters off Sangiang Island.

The operation is one of many challenging ones that Galih has faced during his decade-long career at Basarnas.

"I have seen terrible accidents on the job, such as the Superjet crash at Mount Salak in 2012. I am thankful I can help people in distress," he said, referring to the crash of a Sukhoi Superjet into the side of Mount Salak in Indonesia, which killed all 45 people on board.

"I pray that the search team will be given health and strength. We may be tired but this work is our responsibility and it must be done."