Video released by South Korea's navy shows divers trying to smash a window to get inside the wreck of the ferry that sank last week.
The divers can see just a few centimetres ahead.
If they use the oxygen lines like those seen in the video they can work for up to an hour at a time.
When they use the more back-mounted oxygen tanks they only have about 20 minutes before the alarm to resurface goes off.
The footage, shot on April 21, five days after the disaster, also shows the night search operation.
The ferry capsized and later sank on a routine trip from the port of Incheon near Seoul to the island of Jeju.
Of the 476 passengers and crew on board, 339 were pupils and teachers from Danwon High School in Ansan.
Only 174 people were rescued.
By late on Wednesday the number of confirmed dead stood at 150.
All those still missing are presumed dead.
The captain and two crew members have been arrested on charges of negligence.
Underwater video of ferry search released (1:04)
April 23 - South Korea's navy releases underwater video of search operations those still missing from the sunken ferry. Paul Chapman reports. ( Transcript )