The presumed captain was picked up from an oil rig on Wednesday, said Lt. Col. Tony Syaiful.
"We are still investigating him because he is a key witness,'' said Syaiful, declining to say where the man, identified as Wiranto, was being treated.
Syaiful said the 15 were well and resting in a health clinic on a small island in the Java sea.
The rescue Thursday brings the number of survivors in the disaster to nearly 220, according to rescue officials and a count by The Associated Press.
The ferry Senopati Nusantara sank after being pounded by heavy waves for more than 10 hours as it neared the end of a two-day journey from the Indonesian section of Borneo island to the country's main island of Java. Officials said the bad weather caused the sinking.
Indonesia's tropical waters are generally between 72 and 84 degrees Fahrenheit. People have been known to survive for days at sea, but only with a buoyancy aid.
One 35-year-old survivor was picked up by fishermen and taken to hospital with chest pain and respiratory problems after drifting in a life raft for four days.
"Six among us died, one by one," said Susilo, who like many Indonesians uses one name. Some of those who died drank sea water, he said.
An additional 27 survivors were rescued elsewhere off the Java coast Tuesday and taken to the eastern port of Surabaya.