The tourist boat carrying around 100 Thai Muslims capsized on Sunday, killing at least 15 people, officials said on Monday.
The accident happened on the Chao Phraya River in Ayutthaya, a UNESCO World Heritage site located some 80 km (50 miles) north of the Thai capital of Bangkok.
The double-decker tourist boat with people on a pilgrimage tried to overtake a sand barge and ran into a barrier, police said, causing the boat to capsize.
Eleven people are still missing and police divers resumed their search early on Monday. No foreigners were believed to be among the dead.
According to the officials, the boat was overcrowded. It has 50 passengers over its capacity at the time of the accident, police commander Surapong Thampitak told Reuters on Monday.
The boat's driver will be charged with reckless driving causing loss of life and overloading the boat beyond its safety limit, said Surapong.
The driver was detained on Sunday night and was being questioned by police, he said.