The vessel was traveling from Lanto village in Buton Central regency, Southeast Sulawesi province, to nearby Lagili village when it capsized just after midnight on Monday, as reported by the local search and rescue agency head, Muhammad Arafah.
The wooden boat was carrying 40 people, despite being designed for just 20.
Rescuers were searching on Monday for the 19 people still missing in rough seas, while they had recovered 15 bodies, and six people were rescued, according to Arafah.
To aid in the search for the missing people, three inflatable boats, two fishing boats, and six divers were deployed, he added.
Thousands of residents had traveled to their villages to celebrate the regency's 9th anniversary on Sunday, and many people were transported by fishing or passenger boats.
Television news showed footage of people on fishing boats retrieving bodies in the overnight darkness, as well as grieving relatives waiting for information at a port and a local hospital.
Indonesia is an archipelago with over 17,000 islands, and ferries are a common means of transportation. Due to lax safety standards and problems with overcrowding, accidents occur frequently.
In 2018, an overcrowded ferry with about 200 people onboard sank in a lake in North Sumatra province, resulting in the deaths of 167 people.
In one of the country's worst recorded disasters, an overcrowded passenger ship sank in February 1999 with 332 people aboard, with only 20 people surviving.