Despite waves up to three metres (nine feet) and near gale-force winds, teams were still trying to search the ferry.
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 news agency, citing one rescue worker, said divers were having to blindly stick their hands into clumps of floating objects to fumble for bodies.
On the surface recriminations continue, 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.