The 10-year-old had been travelling with his family in Vietnam’s famed Ha Long Bay when the vessel they were sailing on became caught in a sudden storm. Trapped in the boat as it capsized, the boy found refuge in an underwater air pocket, where he stayed and waited for help.

He was eventually found by rescue teams relatively unharmed, although he was suffering from mental distress. He was warmed up and transferred to a nearby hospital, local outlet VietnamNet reported.

“Everything happened so fast,” the boy said. “I tried to get out, and then the soldiers rescued me.”

At least 35 people are confirmed dead from the sinking, state media reported, revising down an earlier toll but warning that it could still rise further. Vietnam News Agency said there were 53 passengers and crew on board and 10 people had been rescued. Four people remain missing.

Another survivor, a 36-year-old Vietnamese man, described escaping the overturned boat by dumping his life jacket and swimming through an underwater window.

Fire extinguisher salesman Dang Anh Tuan described the moment the vessel suddenly capsized during bad weather, leaving little time for those on board to react.

“It rained for about 15 minutes, and then the boat started to shake vigorously, tables and chairs were jostled around and seconds later the boat overturned,” Tuan told the Associated Press.

He said that the boat quickly filled with water and he lost all orientation.

“I tried to breathe. But more water came in. I took a deep breath, got rid of my life vest and dove down. I saw a streak of light and followed it to swim out, escaping the boat, and then I climbed on the overturned boat to look for help,” he said.

Tuan, along with several others who had been on board, clung onto the overturned boat and waited for another two hours in pouring rain until emergency crews arrived.

He’d been holidaying in Vietnam’s popular tourist bay with 11 university friends. Only three of their group survived.

VN Express reported that the Wonder Sea was caught in a storm around 1:30pm local time (4:30pm AEST), which caused it to capsize.

Most of the passengers were from the Vietnamese capital Hanoi and travelling as families, according to VN Express, which reported the eldest passenger as 53 years old and the youngest as just 3.

Over 20 children were on board, VNExpress said.

The capsize reportedly happened near the Dau Go Cave – one of the largest in the bay.

Halong Bay is a popular tourist hotspot and a UNESCO world heritage site comprising about 1600 limestone islands and islets.