The vehicle was travelling on a bridge on Tuesday in Anshun, Guizhou province when it crashed through a roadside barrier and fell into the water.
Those on board reportedly included students who were scheduled to take the "gaokao" university entrance exams, state media said.
The cause of the crash is currently under investigation.
The driver, who is among the dead, had wokred on this bus route since 1997, Xinhua said.
It's unclear how many people were on the bus at the time and how many of those were students.
The bus was removed from the water on Tuesday afternoon.
Guizhou Fire Department confirmed on its Weibo page late on Tuesday that the search and rescue effort was continuing. About 141 fire rescue personnel, including 19 divers are currently involved in the effort, according to the fire department.
Other rescue organisations including the police and public security are also involved in the operation.
Video footage from the incident shows the bus moving slowly on the road before making a sudden left turn, crossing over six lanes of a dual carriageway, breaking through the safety barrier and crashing into the water.