Kapit District Council member Daniel Levoh said he received information a pregnant woman was the lone casualty but police said they hadn't verified the report, Bernama reported.
The express boat was making a 40-minute trip in Rajang River to Kapit and Sibu when it crashed, officials said.
Levoh said the Bakun Mas Express Boat crashed onto a rock and passengers swam to shore. Most of the passengers were timber and oil palm plantation workers heading for the Gawai Dayak festival celebrated by Sarawak's indigenous Dayak people.
He said the express boat was believed to have more than 100 passengers even though its official load capacity is for 67 seated passengers.
"The boat was supposed to have made the normal transit stop at Belaga but it did not do so as it was already packed," he said. "Many passengers were seen sitting on its roof which is not allowed."