FIVE people have died and one person remains missing after a whale watching boat with 27 people on board sank off Vancouver Island, Canadian authorities said.

The vessel made a mayday call late Sunday afternoon on a calm, clear and sunny day off the tourist community of Tofino, a popular destination for whale watchers, the Joint Rescue Coordination Centre said. The cause of the sinking remains a mystery.

A search by the rescue agency staffed by Canadian military and Coast Guard personnel concluded late Sunday with five people dead, 21 rescued and one person missing, said Lt-Cmdr Desmond James, a spokesman for the rescue agency.

The Royal Canadian Mounted Police (RCMP) have taken over the search for the person who remains missing.

Boats from the nearby Ahoushat First Nation arrived first on the scene, said aboriginal councillor Tom Campbell. He was on the waterfront and watched as rescuers brought several survivors ashore.

"Their looks tell the whole story," he said. "You can't describe looks on people that are lost. They look totally lost - shocked and lost."

The boat, the 65ft Leviathan II, was partially submerged eight nautical miles west of Tofino.

"As of right now the case has been turned over to the RCMP and it's a missing person file," said Lt-Cmdr James. "Fortunately, we did manage to rescue 21 people."

Tofino's mayor described the town's mood as tense but commended residents for their quick aid in the rescue effort.

"Everybody's heart is just breaking for what's going on here and wanting to be as helpful as possible," said Josie Osborne.

John Forde, who runs The Whale Centre, another whale watching operation, responded to the call for help and was told the search was for four or five missing people.

The ship that went down was operated by Jamie's Whaling Station, Mr Forde said. He said he had no idea how it could have sunk.

"Over the course of a season and years we take out thousands and thousands of people on these trips in conditions similar to today," Mr Forde said. "I have no idea what the issue was or what actually happened."

Mr Forde said Jamie's Whaling Station was one of the first of its kind off Vancouver Island and had been around for many years.

Tofino fishing guide Lance Desilets said at least 12 rescue boats were already out on the water when he arrived on the scene after responding to the call for help.

"I saw a lot of personal belongings, a long diesel slick and the top 10 feet of the Leviathan II sticking out of the water," Mr Desilets said. "It's a sad day for our community."

Many of the survivors were taken to Tofino General Hospital and some were already discharged on Sunday night, said Valerie Wilson, a spokeswoman for the Vancouver Island Health Authority.

Canada's Transportation Safety Board said it was investigating the boat's sinking.