The 53-year-old diver, known by his surname Lee, lost consciousness shortly after diving into waters about 80 feet deep. Fellow divers realized Lee ceased communication and then pulled him to the surface. He was taken to the hospital where he later died.
South Korean Prime Minister Chung Hong-won has since required that government officials overseeing the search thoroughly check divers' health conditions before they begin their dives.
Lee was a veteran of Undine Marine Industries and specialized in engineering and rescue work. Doctors are unsure about the cause of death but say his tests came back normal excepting a high amount of potassium registering in his blood test.
There are 130 divers searching for the bodies from the ferry who face obstacles in their search such as debris blocking cabin doors. They recently put up extra nets around the search area to prevent any of the 40 missing bodies from floating away.