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.
