Most of the victims of the tragedy on the Kasai river were students on holiday, local administrator Jacques Mbila told AFP.
The main reasons for the sinking were "overcrowding and drunk drivers", Mbila added.
The accident happened overnight Thursday to Friday on the river Congo in the Idiofa area in the southwest of the vast country.
Local radio journalist Nelly Mafuta said she saw two nuns among the 27 dead, and the Idiofa cathedral said it held a mass in tribute to the victims.
The boat had left Dibaya in the restive central Kasai province on Thursday, heading northwest for its final destination of Ilebo more than 500 kilometres (300 miles) away.
Such transport is one of the most used in DR Congo with its numerous waterways including the Kasai river, the country's second longest after the mighty Congo at over 2,360 kilometres.
Boats frequently capsize in DR Congo, most often due to taking on too many passengers, the lack of safety measures and inexperienced helmsmen.
Violence in the Kasai region erupted last year after a tribal chieftain known as the Kamwina Nsapu openly challenged the authority of President Joseph Kabila's government and was killed in a police operation in August 2016.