27 people on board at time of accident, 18 sent to hospital.
The boat carrying 27 people sank near Tofino on Vancouver Island in the west of the country, coastguard and rescue officials said.
A coroner quoted by Canadian media said at least five people had died. Their nationalities are unknown. A hospital spokeswoman said 18 people had been rescued.
Valerie Wilson, of the Vancouver Island Health Authority, told Associated Press that the survivors had been admitted to Tofino General Hospital. three of whom had since been transferred. The search is expected to continue throughout the night. The first rescue boats to arrive belonged to a nearby First Nation community, councillor Tom Campbell said. "Their looks tell the whole story,'' he told AP. "You can't describe looks on people that are lost. They look totally lost - shocked and lost.''
The boat appeared to have sank some distance from the shore, the Canadian broadcaster CBC said. Canadian media said the 20-metre (65ft) boat, Leviathan II, was operated by Jamie's Whaling Station and Adventure Centers. The company's website said its whale-watching tours were finishing for the season on 31 October, and that it had been operating for 30 years. Reports said seas near Tofino were calm and the weather sunny when the boat sank. Tofino is a popular destination for tourists wanting to spot humpback and Pacific Gray whales, and trips usually last up to three hours.