JINDO, South Korea (Reuters) - The death toll from a capsized South Korean passenger ferry rose to 25 on Friday as parents of missing schoolchildren blamed the ship's captain for the tragedy after he and shipping company officials made emotional apologies for the loss of life.

Of 475 passengers and crew on the Sewol ferry, which capsized in calm seas on Wednesday, 179 people are listed as safe and 271 are still missing.

South Korean authorities were due to restart rescue efforts on Friday morning and to deploy an unmanned submarine to inspect the vessel. Divers, hampered by strong tides and murky waters, have been unable to gain access inside the ferry.

Theories about the cause of the accident swirled and an official investigation was due to resume with questioning of the captain. The vessel started to sink on Wednesday on a routine trip out of the major port of Incheon to the holiday island of Jeju, 480 km (300 miles) to the south.

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. Relatives of those who died have accused him and some of his crew of being among the first to leave the vessel.

Both 69-year-old Lee and the company that owns the ship have apologized for the loss of life, although neither has admitted responsibility.

Most of those on board were children from a high school in the suburbs of Seoul who were on a field trip to Jeju.

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 center. 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.

Witness accounts say crew members instructed some passengers to remain where they were as the ship listed sharply and then capsized in about two hours 25 km (16 miles) southwest of Jindo, a large South Korean island connected to the mainland.

HUNTING FOR CLUES

South Korean media have reported the ship may have turned before listing sharply to port, but investigators have declined to comment.

While Lee and some of the crew members have been criticized for their role in the sinking, he was described as a "veteran" by Chonghaejin Marine Co Ltd, the owner of the vessel.

"He is a veteran captain who has run such passenger ferry between Incheon and Jeju for 20 years," said Song Ki-chae, who heads one of Chunghaejin's branch offices.

Song paid tribute to Park Ji-young, one of the crew members who died after escorting children to safety and helping them don life jackets.

Of the 20 crew, half are not accounted for.

Officials will investigate Chunghaejin, the unlisted operator, which owns four other vessels, and which reported an operating loss of 785 million won ($756,000) last year.

Earlier efforts to locate survivors inside the hull, which is still partly above water, did not succeed. Data shows that the speed of the underwater current varies throughout the day and, at its strongest, hit 10km/h, making diving impossible.

Although the water at the site of the accident is relatively shallow at under 50 meters (165 feet), it is still dangerous for the 150 or so divers working flat out, experts said. Time was running out to find any survivors trapped inside, they said.

"The chances of finding people in there (alive) are not zero," said David Jardine-Smith, secretary of the International Maritime Rescue Federation. However, he said 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."

(Writing by David Chance; Editing by James Pearson, Mark Bendeich and Paul Tait)

South Korea