Local reports said a logging vessel not authorised to take passengers went down in bad weather on Wednesday.
DR Congo Red Cross president Dominique Lutula told a radio station about 270 people had survived.
But he said that some people were still missing, with bodies thought to be trapped inside the sunken vessel.
The boat – two logging barges linked together – went down on Lake Mai-Ndombe, in Bandundu province, some 400km (
Mr Lutula said 73 bodies had been found so far on the shores of the lake.
"There are still some people missing. But we don’t know how many because there was no ship’s manifest," he said.
Red Cross workers were recovering bodies and rounding up survivors, he added.
Lake Mai-Ndombe is 120km long and 50km wide at it broadest point.
River transport is widely used on DR Congo’s waterways but accidents happen frequently because of overcrowding and poor vessel maintenance.
Source: BBC News