JINDO, South Korea (AFP) - Divers battled strong currents and murky waters on Friday to finally enter a South Korean ferry two days after it sank, as investigators revealed the captain was not at the helm when the vessel capsized with hundreds on board.

The unfolding tragedy was compounded by the apparent suicide of a high school vice-principal, who had been rescued from the 6,825-tonne Sewol that sank on Wednesday morning with hundreds of his students trapped inside.

More than 48 hours after the ferry suddenly listed and capsized, exhausted dive teams - who waded through powerful currents - finally managed to access the interior.

After several attempts, two divers managed to pry open a door and enter the cargo section, a senior coast guard official said, briefing relatives of the missing. Hours later, another two-man team accessed one of the cabins, but found nothing.

"The search operation will continue through the night," the official said.

"Visibility is almost non-existent. You can hardly see your hand in front of your face," said one diver when he returned to the harbour at nearby Jindo island.

The confirmed death toll stood at 28, but the focus of concern remained on the 268 still unaccounted for.

Of the 475 people on board when the Sewol capsized, only 179 were rescued and no new survivors have been found since Wednesday.

Three giant, floating cranes reached the rescue site, but regional coastguard commander Kim Soo Hyun stressed they would not begin lifting the multi-deck ferry until they were sure there were no survivors inside.

"I want to be clear: There won't be any salvage work done against the will of the families," Mr Kim said.

More than 350 of those on board were from the Danwon High School in Ansan city just south of Seoul.

Among the parents of those students still missing there was bitter resentment at what they saw as the inadequacy of the official response.

"It's been two days but no one has been brought out alive," complained Mr Lee Yong Gi, whose son was among the missing students.

"I firmly believe that the kids are alive. We need to rescue them as soon as possible. But officials are dragging their feet," Mr Lee told AFP.

Another father accused the authorities of indifference and deception in an appeal broadcast live on television.

"The government lied yesterday," he said, speaking from a podium in a Jindo gymnasium where hundreds of blanket-wrapped relatives have been sleeping on the floor since the tragedy unfolded.

Disputing the official figures of hundreds of divers, vessels and aircraft being deployed, he said he and other relatives had visited the rescue site and seen only a dozen ships and helicopters.

"Everyone, is this the reality of South Korea? We plead once more, please save our children," he said.

The local police said they on Friday found the body of Danwon High School vice-principal Kang Min-Kyu, who had managed to escape the Sewol as it sank.

The police said the cause of death was under investigation, but sources cited by local media said he was found hanging by his belt from a tree next to the gymnasium.

The initial public backlash has centred on the captain Lee Joon Seok, and his 28 crew members, most of whom survived the disaster.

State prosecutors said preliminary investigations showed the third officer was at the helm of the ferry.

"The captain was not in command when the accident took place," prosecutor Park Jae Eok told a press briefing.

The captain had apologised on Thursday to the victims and their relatives, but offered no clear explanation for what caused the Sewol to capsize.

Tracking data from the Maritime Ministry showed that the ferry made a sharp turn just before sending its first distress signal.

Some experts believe a tight turn could have dislodged the heavy cargo manifest - including more than 150 vehicles - and destabilised the vessel, causing it to list heavily and then capsize.

But others suggested the turn might have been caused by a collision with a rock or other submerged object.

Chief prosecutor Lee Seong Yoon stressed there was "no limit" to the range of the investigation.

"We will make sure... those responsible are sternly held accountable," Mr Lee said.

As well as the cause of the disaster, investigators will be looking at why passengers were ordered to stay in their cabins and seats for up to 40 minutes after the ferry ran into trouble.

Furious relatives believe many more people would have escaped if they had reached evacuation points before the ship listed sharply and water started flooding in.

Newspaper editorials were scathing with the Dong-A Ilbo daily calling the rescue response "ludicrous".

"We have the world's finest shipbuilding industry in the 21st century, but our mindset is in the 19th century," the newspaper said.