Rowers on board the two long, narrow kayak-like boats were practising for a race in Taohua River in the city of Guilin when the accident occurred Saturday afternoon, tipping 57 people into the water.
Some 40 people have been pulled out of the water alive and rescue work continues, Xinhua said.
Authorities in Guilin said villagers had organised a practice session without notifying police, and that two organisers were detained.
Races are held nationwide every year to mark the dragon boat festival, which falls on the fifth day of the fifth month of the Chinese lunar calendar, or the summer solstice.
The festival pays tribute to the ancient poet Qu Yuan who, legend has it, drowned himself in protest against being falsely accused of treason.