At least four people died, according to the Joint Rescue Co-ordination Centre. It wasn't immediately clear how many people survived.
"There are fatalities involved, and survivors," the Joint Rescue Co-ordination Centre said in an earlier recorded message.
"We're also conducting a search to confirm that we've recovered everybody," the message said.
The boat was a whale-watching vessel, according to the Canadian Broadcasting Corporation.
A witness told CBC that 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 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.
The boat is named Leviathan II and is owned by Jamie's Whaling Station & Adventure Centres, according to the Joint Rescue Co-ordination Centre.
Search coordinators have requested a military helicopter and a fixed wing aircraft to join in the search. Two Canadian Coast Guard vessels have deployed to Tofino, as well as other vessels.
The search continued after nightfall, the coordination center said.
The Transportation Safety Board of Canada will investigate the sinking, it said in a statement.
CNN reached out to the Jamie's Whaling Station but did not receive immediate response.