South Korean officials have said the recovery operation after Wednesday's ferry disaster may take two months.
Divers are battling strong currents in poor visibility to reach 270 missing passengers - 32 are confirmed dead. Some 174 passengers have been rescued.
Relatives have begun providing DNA samples to help identify the dead.
Ferry captain Lee Joon-seok, 69, and two crew members have been arrested. Lee says he delayed the evacuation fearing passengers would "drift away".
He faces charges including negligence of duty and violation of maritime law.
Divers saw three bodies in the ship on Saturday but were unable to retrieve them.
Asked how long the rescue operation was likely to continue, Shin Won-Nam, the head of the Emergency Management Centre, told reporters that it could take weeks, if not months.
"We are not sure about it. But according to the experts, the rescue may last one or two months," he said.
He added that it was very unlikely someone trapped alive after the sinking could survive if it took this long.
Investigations are focusing on a sharp turn the vessel took before it started listing and whether an evacuation order could have saved lives.
Footage from the ship appeared to show instructions from crew members for passengers to remain on board even as it tilted dramatically to one side.
The ferry Sewol was sailing from Incheon, in the north-west, to the southern resort island of Jeju. It capsized and sank within two hours.
Lee, who had already been questioned by police, was shown on television today after his arrest.
"I am sorry to the people of South Korea for causing a disturbance and I bow my head in apology to the families of the victims," he said.
Some experts believe the ship's tight turn could have dislodged heavy cargo and destabilised the vessel, while others suggest the sinking could have been caused by a collision with a rock.
Messages and phone calls from those inside painted a picture of people trapped in crowded corridors, unable to escape the sharply-listing ferry.
Officials say air has been pumped into the ship to aid any people trapped inside and to help refloat the vessel.
But they say cranes at the site will not be used until they are certain no-one inside is alive.
NETS AROUND FERRYT
The South Korean coastguard said today that a civilian diver had seen three bodies through a window in the fourth floor of the ship.
The diver was not able to retrieve the bodies because of floating objects and time restrictions on diving, the coastguard said.
But three other bodies found in the water near the wreck were recovered, bringing the number of confirmed dead to 32.
Choi Sang-hwan, deputy director of the national coastguard, said nets would be placed around the sunken ferry to prevent any bodies drifting away.
Some 350 of those on board were students from Danwon High School in Ansan, a suburb of Seoul, who were on a school outing when the ferry sank.
Hundreds of relatives of those on board have been camping at a gymnasium on Jindo island near the scene of the disaster.