The boat was carrying 27 people when it sank near Tofino on Vancouver Island, according to the coastguard. Sea conditions had been reported as being calm at the time, the BBC said.
Twenty-one people were rescued, emergency officials said, while one person is still missing.
The nationalities of the victims are still unclear.
The accident happened after the 65ft boat, Leviathan II, made a mayday call late on Sunday afternoon, the Joint Rescue Co-ordination Centre said.
Survivors were reportedly admitted to hospital, with three of them since transferred elsewhere.
In a statement on the boat operator’s website, Jamie Bray, owner of Jamie’s Whaling Station and Adventure Centres said: "It has been a tragic day. Our entire team is heartbroken over this incident.
"We are doing everything we can to assist our passengers and staff through this difficult time. We are co-operating with investigators to determine exactly what happened."
Tofino is a popular destination for tourists wanting to spot humpback and Pacific Gray whales, with trips usually lasting up to three hours, the BBC said.
Update - 13.24
The foreign secretary Philip Hammond has confirmed that five British nationals died on board the whale watching boat.
