Rescuers entered the interior of the sunken ferry Sewol on Friday, the third day of their frantic search for survivors.
Navy divers first secured a route for their fellow divers to enter the ship's dining facility, where many of the victims were thought to have been trapped. They plan to begin their search of the dinning room at 3 p.m.
Rescuers also succeeded in injecting air into the ship to help possible survivors breathe.
Military personnel also installed buoys to keep the ship afloat, officials said.
The weather was fairly mild on Friday. For the first two days of the rescue efforts, divers had difficulty operating underwater due to strong tidal currents and poor visibility.
The Defence Ministry said that the rescue team had formed five teams -- each with two divers -- and sent them underwater one by one. It said that many divers could not be sent all at once due to the risk of an accident.
Experts say that it takes 15-20 minutes for a diver to reach the ship, 10-25 minutes for their search and 15 minutes to return to the surface.
"The divers can go underwater, holding onto a life line. If they go down there, they cannot see anything well, and due to the fast movement of the water, an accident could happen should they lose the life line," Defence Ministry spokesperson Kim Min-seok told reporters.