A civilian diver involved in searches for dozens of missing people from the South Korean ferry disaster has died, as other divers helped by better weather and easing ocean currents were picking up efforts to retrieve more bodies from the sunken ship.

The Sewol carried 476 people, most of them students from a single high school near Seoul, when it sank off South Korea’s southern coast on April 16. Only 174 survived, including 22 of the 29 crew members. The sinking left more than 260 people dead, with about 40 others still missing.

On Tuesday, a civilian diver died at a hospital after becoming unconscious, government task force spokesman Ko Myung-seok said in a statement. He is the first fatality among divers mobilised following the ferry’s sinking, according to the coast guard.

The 53-year-old diver was pulled to the surface by fellow divers after losing communication about five minutes after he began underwater searches, Ko said. It was his first search attempt, Ko added.

In searching for the missing, divers have been working their way into the last three unopened rooms, next to a snack bar on the ferry’s third floor, Ko earlier told reporters.

He said the search team does not expect to find many bodies in those rooms as they were not assigned to the high school students who made up most of the ferry’s passengers. The divers will revisit areas searched earlier, while checking other areas such as bathrooms on each floor, looking for more victims. Darkness, floating debris and the maze of corridors and cabins on board have made the search difficult.

Investigators have also made their first arrests of people who were not on board the Sewol when it sank. The three people arrested are suspected of negligence in their handling of cargo on the vessel.

In all, 19 people have been arrested in the investigation, 15 of them crew members accused of abandoning passengers. An executive with ties to Chonghaejin, the company that owns the ferry, was arrested on suspicion of malpractice related to company finances.

Improper stowage and overloading of cargo is suspected as a possible reason the ferry sank. The ferry was carrying an estimated 3608 tons of cargo, more than three times what it could safely carry. A ferry loaded too heavily could lose its balance making even a small turn.