The exact number of people on board is not clear but authorities said the vessel was loaded with several manned vehicles including trucks, cars and motorcycles.
"Based on the accounts by family relatives, about 30 people are still missing," Lampung provincial search and rescue chief Saidar Rahman Jaya told AFP.
"Today is the third day since the accident. The possibility of them surviving is very slim. Our priority is to search for bodies believed to be trapped in their vehicles," he said.
About 78 vehicles remained inside the ferry which sank, Jaya said, adding that up to 130 rescuers including navy divers were involved in the operation.
Such accidents are common in Indonesia, where ships and boats have a poor safety record and where the 240-million population is scattered over a vast archipelago of more than 17,000 islands.