Coastguard, navy and private divers scoured the site of the accident, about 20km (12 miles) off the country's southwestern coast.
Earlier, rescue teams hammered on the hull of the upturned, mostly submerged vessel, hoping for a response from anyone trapped inside, but they heard nothing, local media reported.
The vessel, which was carrying 475 passengers and crew, capsized on Wednesday during a journey from the port of Incheon to the holiday island of Jeju.
Coastguards recovered five more bodies late yesterday, raising the death toll to 14 people. Another 179 passengers have been rescued, leaving 282 unaccounted for and possibly trapped in the vessel.
One parent, Park Yung-suk, whose daughter was one of 340 children and teachers from Danwon High School in Seoul on board the vessel, said she had seen the body of her teacher brought ashore.
The captain of the ship, Lee Joon-seok, 69, faces a criminal investigation amid unconfirmed reports he was one of the first to jump to safety from the stricken vessel.
One official said authorities were investigating whether the captain had abandoned the vessel early, and one of the charges he faced was violating a law that governs the conduct of shipping crew.
Many survivors told local media Mr Lee was one of the first to be rescued, although none actually saw him leave the ship. The coastguard and the ferry operator declined to comment.
Although the water at the site of the accident is relatively shallow at less than 50 metres (165 feet), experts said it is still dangerous for the 150 or so divers working flat out, and added that time was running out to find any survivors trapped inside.
"The chances of finding people in there [alive] are not zero," said David Jardine-Smith, secretary of the International Maritime Rescue Federation, buy he added that conditions were extremely difficult. "There is a lot of water current and silt in the water, which means visibility is very poor and the divers are basically feeling their way around."
The government said it was not giving up on the possibility of finding survivors, while the coastguard also turned its attention to what may have caused the disaster in calm seas.
"Today, we began looking into the cause of the submersion and sinking ... focusing on any questions about crew negligence, problems with cargo holding and structural defects of the vessel," said senior coast guard official Kim Soo-hyun.
There has been no official explanation for the sinking, although officials have denied reports the ship, built in Japan 20 years ago, was sharply off its authorised route.
Although the wider area has rock hazards and shallow waters, they were not in the immediate vicinity of its usual path.
The ferry was found to have three safety deficiencies in 2012, including one related to navigation, but passed subsequent safety checks in 2013 and 2014, according to international and Korean shipping records.
Its capacity was increased to more than 900 people from 800 when it was imported from Japan in late 2012, shipping sources said, but the expansion passed all safety tests.
State broadcaster YTN quoted investigation officials as saying the ship was off its usual course and had been hit by a veering wind that caused containers stacked on deck to shift. It was listing heavily to one side on Wednesday as passengers wearing life jackets scrambled into the sea and waiting boats.
It sank within about two hours and witnesses and media showed two life rafts from the ship successfully inflated and launched. Earlier reports said just one had inflated.
The operator, Chonghaejin Marine Co Ltd, based in Incheon, came under sharp criticism after its officials, for the second day, avoided many questions posed about the conduct of the captain and crew.
The unlisted operator, which owns four other vessels, reported an operating loss of $756,000) last year.