(Adds diver comments)
By Jungmin Jang and Ju-min Park
MOKPO/JINDO, South Korea, April 18 (Reuters) - Strong tides and murky waters off South Korea's southwest coast were hampering efforts on Friday to find survivors from a ferry accident over 48 hours previously that left hundreds of passengers missing, most of them high school students.
Local media said the Sewol ferry may have made a sharp turn during its journey on Wednesday, which caused its cargo to shift and the boat to list sharply and begin to sink.
Investigators declined to comment on the reports.
Twenty eight passengers are officially listed as dead, 179 have been rescued and 268 are missing, presumed trapped in the stricken vessel, out of 475 passengers and crew bound from the port of Incheon to the holiday island of Jeju.
Many of the missing are children from a school on the outskirts of Seoul and hopes are fading that any will be found alive.
"We cannot even see the ship's white colour. Our people are just touching the hull with their hands," Kim Chun-il, a diver from Undine Marine Industries, told relatives gathered near the site of the rescue effort in the port city of Jindo.
Kim said that two divers had to return to the surface when an air pump stopped and said strong tides were impeding the rescue.
Rescuers have pumped air into the vessel, but divers have not yet entered areas of the ship where many of the missing are believed to be.
Coastguard officials have said the investigation was focused on possible crew negligence, problems with cargo stowage and structural defects of the vessel, although the ship appears to have passed all of its safety and insurance checks.
The captain, Lee Joon-seok, faces criminal investigation, which is standard procedure in South Korea.
Both the 69-year-old Lee and the company that owns the ship have apologised for the loss of life, although neither has admitted responsibility.
Investigators said that Lee may not have been on the ship's bridge at the time of the accident and the vessel was being steered by the third mate, a normal situation on many ship journeys.
The ferry went down in calm conditions and was following a frequently travelled 400 km (300 mile) route in familiar waters.
RELATIVES QUESTION CREW LEAVING SHIP
Parents of the missing schoolchildren blamed the ship's captain for the tragedy after he and shipping company officials made emotional apologies for the loss of life.
Some also heckled South Korean President Park Geun-hye when she visited the site on Thursday.
Witnesses have said that the captain and some of the crew left the vessel while others instructed passengers to remain in place as it began to sink.
Relatives were in mourning overnight in a hospital in the city of Mokpo, close to the port city of Jindo, which is acting as a rescue centre. Some of them spoke bitterly of the captain.
"How could he tell those young kids to stay there and jump from the sinking ship himself?" said Ham Young-ho, grandfather of 17-year-old Lee Da-woon, one of the dead.
Lee has not made any public statement on whether or why he may have left the vessel before many of the passengers.
The record of the ferry owner is also under investigation and documents were removed from its headquarters in Incheon on Friday.
Chonghaejin Marine Co Ltd, the owner of the vessel, is an unlisted company that operates five ships. It reported an operating loss of 785 million won ($756,000) last year.
According to data from South Korea's Financial Supervisory Service, a government body, Chonghaejin is "indirectly" owned by two sons of the owner of a former shipping company called Semo Marine which went bankrupt in 1997.
The Sewol ferry is 20 years old and was built in Japan and was acquired second-hand by the Korean operator. (Writing by David Chance; Editing by James Pearson and Raju Gopalakrishnan)