A diver searching the sunken Sewol ferry died Tuesday, according to South Korea's Government Rescue Headquarters.
"A civilian diver, Lee, lost the communication line at 25 meters under the sea five minutes into his first dive," spokesman Koh Myung-suk said.
"By the time his colleagues went to save him, Lee was unconscious and unable to breathe by himself," Koh said.
Lee, whose full name was not provided, was helicoptered to a hospital where he was pronounced dead, the spokesman added.
Park In-ho, head of the hospital where Lee died, said the diver's blood was tested and appeared normal except for high potassium. His chest X-ray also showed no sign of irregularity.
Not including the diver, the death toll in the ferry disaster has risen to 263, with 39 people still missing, the government reported.
Over the weekend, South Korean President Park Geun-hye visited the port where the rescue operation is based to console families and encourage divers.
Nearly 130 divers are combing the ship, looking for the remaining missing bodies.
Ferry survivors honor lost classmates First ship on scene saw no evacuation
The search-and-rescue operation has turned into a grueling recovery of corpses. No one has been found alive since the ferry sank April 16 with a passenger load largely made up of high school students on a field trip.
The work has become even more difficult because divers have faced closed cabin doors blocked by debris.
Corralling the debris has been difficult for search teams.
Mattresses and clothing from the ship have been found up to 9 miles (15 km) away from the accident site, said Park Seung-ki, a spokesman for the rescue operation.
Large stow and trawler nets will be set up around the sunken ship to catch items that may float away, he said. At the same time, some three dozen ships will be clearing an oil spill from the ferry, which is threatening the livelihood of the local fishermen.
The ferry sank en route from Incheon to the resort island of Jeju, off the nation's southwestern coast.