Rolling seas whipped up by strong winds were badly complicating efforts to find the remains of more than 100 people still unaccounted for 11 days after the Sewol capsized with 476 people -- many of them schoolchildren -- on board.
Yesterday four more of the ship's crew were arrested. All 15 surviving mariners are now in custody and face charges ranging from criminal negligence to abandoning passengers.
Despite waves up to three metres (nine feet) tall and near gale-force winds, teams of divers were still trying to get into the ferry.
Heavy seas prevented divers from getting into the ship yesterday.
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Forecasters warned wind and rain would pick up throughout the day.
"There is a possibility that a high wave advisory will be issued in sea areas around Jindo Sunday afternoon," Yonhap news agency quoted a weather service official as saying.
The confirmed death toll from the tragedy stood today at 187, with 115 unaccounted for.
Making up the bulk of the passengers on the 6,825 tonne Sewol when it sank were 325 high school students -- around 250 of whom are either confirmed or presumed dead.
Frogmen -- who were unable to get inside the ferry for the first two days -- have battled strong currents, poor visibility and blockages.
The conditions each mission can be no more than a few minutes in length.
But even in this short time, they are coming across scenes of horror in the murky water, including one dormitory room packed with the bodies of 48 students wearing lifejackets.
Around a quarter of the dead recovered so far have been found in waters outside the sunken vessel, and there are fears that some of the missing may have drifted free from the wreck.