MOKPO, SOUTH KOREA—Koo Bon-hee could see the exit. For half an hour, as the doomed ferry filled with water and listed severely on its side, the crew told passengers to wait for rescuers.

With their breathing room disappearing, the 36-year-old businessman and some of the other passengers floated to an exit and swam to a nearby fishing boat. But 290 of the 475 people aboard — many of them high school students on a class trip — were still missing after the ferry sank Wednesday off the southern coast of South Korea. Six were confirmed dead and 55 were injured.

Early Thursday, divers, helicopters and boats continued to search for survivors. The high number of people unaccounted for — possibly trapped in the ship or floating in the chilly water nearby — raised fears that the death toll could increase drastically.

In photos: Ferry sinking

It was still unknown why the ferry sank, and the coast guard was interviewing the captain and crew. The Sewol, which can hold more than 900 people, set sail Tuesday from Incheon, in northwestern South Korea, on an overnight, 14-hour journey to the tourist island of Jeju.

About 9 a.m. Wednesday, when it was three hours from Jeju, the ferry sent a distress call after it began listing to one side, according to the Ministry of Security and Public Administration.

Passenger Kim Seong-mok told broadcaster YTN that after having breakfast, he felt the ferry tilt and then heard it crash into something. He said an announcement told passengers to not move from their places and that he never heard another about evacuating.

He said he was certain that many people were trapped inside the ferry as water rushed in and the severe tilt of the vessel kept them from reaching the exits.

Koo also complained about the crew’s efforts during the initial stages of the disaster, saying early misjudgments may account for the large number of missing.

In addition to the order not to evacuate immediately, Koo said many people were trapped inside by windows that were too hard to break.

“The rescue wasn’t done well. We were wearing life jackets. We had time,” said Koo, speaking from a hospital bed in Mokpo, where he was treated for minor injuries. “If people had jumped into the water . . . they could have been rescued. But we were told not to go out.”

Oh-Yong-seok, a 58-year-old crew member, told The Associated Press that rescue efforts were hampered by the ferry’s severe tilt. “We couldn’t even move one step. The slope was too big.”

Student Lim Hyung-min told YTN that he and others jumped into the water wearing life jackets and then swam to a nearby rescue boat.

“As the ferry was shaking and tilting, we all tripped and bumped into each another,” Lim said. Once he jumped, the ocean “was so cold . . . I was hurrying, thinking that I wanted to live.”

The water temperature in the area was about 12 degrees Celsius, cold enough to cause signs of hypothermia after about 90 minutes of exposure, according to an emergency official who spoke on condition of anonymity.

Dozens of coast guard and navy divers searched for survivors around the Sewol’s wreckage a little north of Byeongpung Island, about 470 kilometres from Seoul.

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Coast guard spokesman Cho Man-yong said 16 divers could not get inside the ferry Wednesday night because the current was too strong. The water was muddy and visibility was poor, he said, but divers would try again Thursday morning.

“We cannot give up,” said South Korean President Park Geun-hye, after a briefing in Seoul. “We have to do our best to rescue even one passenger.”