[Latest: 21 people were rescued; the five dead are British. BBC News reports "Three were tourists on holiday from the UK, while two were British nationals living in Canada - one woman from British Columbia and a man from Ontario."
The boat sank around 8 miles from shore. An investigation has been launched and divers are searching for the missing man, a 27-year-old Australian.
The five dead Britons are named as; Stephen Thomas of Swindon, who was the youngest of the victims, aged just 18, with Down’s syndrome, and his father, David, 50, who died with him. The eldest of the victims was Jack Slater, 76, the fourth victim was 29-year-old Katie Taylor who resided in Whistler, and the fifth victim was Nigel Hooker, 63, from Southampton.
Early investigations indicate the boat overturned after passengers moved to one side of the boat and a wave crashed into the opposite side. Life jackets were on board but not mandatory wear]
A military rescue helicopter and plane joined the rescue operation.
An investigation into what happened by the Transportation Safety Board of Canada (TSB) will follow.
BBC News reports early Monday that "the 20m (65ft) boat, Leviathan II, was operated by Jamie's Whaling Station and Adventure Centers." The vessel was some way off the coast when it sunk but the seas were reportedly calm and the weather good.
The whale spotting season at Jamie's Whaling Station and Adventure Centers is due to end October 31. The organisation has a good track record for safety and has been operating for 30 years.
In a statement, the company wrote:
"It has been a tragic day. Our entire team is heartbroken over this incident and our hearts go out to the families, friends and loved ones of everyone involved.