Two men in their 20s riding a Jet Ski-type personal watercraft were thrown into the river after the machine turned over.
The incident happened around 10:30 a.m. Tuesday in Drummondville.
One man was rescued and transported to hospital, where he is being treated for hypothermia.
The other has not yet been found.
Witness Michel Hotte said he saw one of the men struggling in the water.
“He was in a state of panic. He was screaming and had his arms in the air. He was hypothermic. I rubbed his feet,” Hotte said.
He said the watercraft floated to the other side of the river.