Twenty-two Singaporeans had been on the boat; 17 were rescued. After completing the 1,500-metre course, the Singaporeans declined assistance and rowed from the finish line to shore, Cambodian officials said.
But their boat was hit by swirling water as they neared the landing pontoon, Nhim Vanda said.
Relatives gathered at Phnom Penh's Calmette hospital, where the bodies were being brought for autopsies.
Singapore's navy had sent an eight-member team to help the Cambodian authorities with their search and rescue efforts.
About 440 colourful dragon boats, mostly from across Cambodia, were racing to celebrate the country's annual water festival, which marks the start of the rice-harvesting season and the time of the year when the Tonle Sap river reverses its flow from north to south.
There have been several fatalities in previous years' races.