RCMP and emergency crews were called at around 6:30 p.m. CT to the single-vehicle rollover along Highway 16, about seven kilometres east of Shoal Lake.
Police say two men from British Columbia were in the 2002 Chevrolet Cavalier when it left the roadway and rolled over, landing upside down in a water-filled ditch.
Cpl. Miles Hiebert told CBC News the 28-year-old driver escaped, but the 26-year-old passenger was trapped inside the submerged vehicle.
Hiebert said within minutes, motorists who saw the collision pulled the car out of the water using a truck and tow rope.
"It's nothing short of heroic — quick thinking was obviously the order of the day," Hiebert told CBC News on Friday.
"People are obviously extremely resourceful and thankfully had the quick wits enough to take some action."
The male passenger was taken to hospital with life-threatening injuries. RCMP did not release his name.
Hiebert credited the passersby with taking "some very quick and decisive action," noting that it generally takes time for emergency crews to go to highway collisions.
"Hopefully they've been able to make a difference and save this young man's life," he said.