South Korea's prime minister resigned Sunday, blaming corruption and "deep-rooted evil" for the sinking of a passenger ferry that left 300 people dead or missing, as anger grows over the bungled response to the tragedy.

Chung Hong-Won admitted he had not been up to the task of overseeing rescue operations after the Sewol capsized with 476 people -- many of them schoolchildren -- on board.

"I offer my apology for having been unable to prevent this accident from happening and unable to properly respond to it afterwards," he said.

"I believed I, as the prime minister, certainly had to take responsibility and resign."

Parents and relatives of the missing and the dead have blasted the response to the sinking, saying delays in launching the rescue had cost lives.

There has also been rage over perceived corruption and lax safety standards that may have led to the disaster, with claims that the ferry was overloaded and the passenger list was inaccurate and incomplete.

"Looking at the latest accident I came to a painful realisation that there is too much deep-rooted evil and corruption in our society," Chung said.

"I hope that such wrongdoings will be rooted out this time so that an accident like this will never happen again."

The role of prime minister is largely ceremonial in South Korea, with the lion's share of executive power vested in the presidency of Park Geun-Hye.

Park has decided to accept Chung's resignation but only after the government finishes the entire rescue and salvage operation, her spokesman said.

In Jindo, the nearest island to the wreck, relatives of the dead and missing were unimpressed at Chung's move.

"So what?" snapped Ji Hyung-Soo. "My son is there in the sea. His resignation will never ease my bitterness and sadness.

"Anybody responsible for this disaster must be punished severely, but the most urgent thing to do now is to recover the bodies as soon as possible. I'm not interested in anything else."

Prosecutors looking to mete out the justice sought by relatives such as Ji raided the offices of state sea traffic controllers in Jeju island on Sunday, the intended destination of the Sewol, and in Jindo.

They seized records of radio communication with the Sewol and surveillance video footage, Yonhap news agency said.

A transcript of conversations released earlier revealed panic and indecision among crew and sea traffic controllers in the crucial final moments, with neither able to make the call to evacuate passengers.

Grim search for bodies

The confirmed death toll from the tragedy stood Sunday at 188, barely changed in two days. A total of 114 people are still unaccounted for, with many bodies believed trapped in the sunken vessel.

Divers were battling decompression sickness, high waves and strong winds in their grim search for corpses in the ferry.

Further complicating their efforts was the increasing depth of the wreck as it slips slowly into the silt of the seabed, making an already dangerous diving operation even more hazardous.

A coastguard spokesman said 98 frogmen were trying to get into rooms on the fourth deck of the 6,825-tonne Sewol, but he warned the operation was hard.

"As the ship has sunk further... the diving depth has also increased to more than 40 metres (130 feet), posing even more difficulties for search efforts.

"A growing number of divers are reporting decompression sickness," he said.

Pressure rises as divers go deeper, increasing the amount of air they breathe from their tanks.

This not only reduces dive time, but also heightens levels of nitrogen in their bloodstream, raising the risk of potentially harmful bubbles forming in body tissue.

Strong underwater currents, poor visibility and waterlogged debris were making conditions inside the ferry treacherous, coastguard chief Kim Seok-Kyun said on Sunday.

"Hallways and cabins are packed with carpets and blankets swollen by water as well as furniture, blocking entry by divers and making search efforts even more difficult," he said.

Yonhap, citing one rescue worker, said divers were having blindly to stick their hands into clumps of floating objects to fumble for bodies.

All crew in custody

On the surface recriminations continued, with four more of the ship's crew arrested on Saturday.

Their arrest means all 15 surviving crew members responsible for sailing the vessel are in custody, facing charges ranging from criminal negligence to abandoning passengers.

Prosecutors have also raided a host of businesses affiliated with the ferry operator, the Chonghaejin Marine Company, as part of an overall probe into corrupt management.

The widening investigation has seen travel bans imposed on eight current and former executives of the Korea Register of Shipping -- the body responsible for issuing marine safety certificates.