Five people died and 21 were rescued, authorities said. Those killed were all British nationals, according to the British Columbia Coroners Service. They ranged in age from 18-76.
Among those rescued from the water, four people were in stable condition at area hospitals, according to a spokeswoman for the Vancouver Island Health Authority.
"Their looks tell the whole story," Councillor Tom Campbell, with the Ahousaht First Nation, told the Canadian Broadcasting Corporation.
He reportedly watched as rescue workers brought survivors ashore in Tofino.
"You can't describe looks on people that are lost. They look totally lost — shocked and lost," CBC reported Campbell said.
The boat, owned by Jamie's Whaling Station & Adventure Centres
, was a whale-watching vessel named the MV Leviathan II.
The cause of the accident is not clear. It's under investigation by the Transportation Safety Board of Canada.
"The safety and security of our passengers is our main concern and we are absolutely devastated by what took place on the water yesterday," said Corene Inouye, director of operations at Jamie's Whaling Station.
She said the skipper of the Leviathan II has more than 20 years whale-watching experience; the other two crew have five years and three years experience.
"We're still just beginning to get the details from what has occurred yesterday and our first priority is doing everything we can to assist our passengers crew and the families of those who have been impacted by this tragic incident," Inouye said.
Far from shore
A witness told CBC the boat sank far enough from shore that it could not be seen.
"You could see the smaller boats going back and forth to try and help bring people back to shore," Rami Touffaha told the CBC. "The waters weren't choppy so I don't see what could have caused the boat to sink, but you never know in these waters unfortunately," he said.
Another tour operator, John Forde, was on a boat excursion with tourists when he heard the news, the CBC reported. He set course for the Leviathan II and arrived as the boat was nearly submerged.
"It was quite close to the rocks and you could still see part of the vessel above water," he said.
'A tragic day'
The Leviathan II is a 65-foot cruiser, Jamie's Whaling Station said on its website. It has three viewing decks -- one upper and one lower and one in the back of the boat. It seats 46 passengers comfortably, the website said.
The company expressed sorrow over the tragedy in a statement on its website.
"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," said owner Jamie Bray.
He told reporters Monday that the Leviathan II had run the same trip for two decades, twice a day.
"This vessel's operated for 20 years with an absolutely perfect safety record and this is something just totally out of the blue," Bray said.
In 1998, another whale-watching boat belonging to Jamie's Whaling Station was involved in a fatal incident, according to the safety board.
That year, a boat operator and a passenger aboard the boat Ocean Thunder were tossed overboard and drowned in turbulent waters during a whale-watching trip, the board said.