Marilène Gill, a member of parliament who represents part of Quebec, called the deaths a “tragedy.”
“My mother’s heart is with you and my whole team is available,” she wrote on Twitter.
The four minors, all over the age of 10, were found unconscious early Saturday and brought to the local community health center, where a doctor pronounced them dead.
Rescue teams are still searching for the missing man, who is in his 30s. Divers from the provincial police service, police on all-terrain vehicles and boats, the Canadian Armed Forces and a Coast Guard helicopter are helping to scour the shoreline and river.
The tides in the area can be dangerous, Canadian news media reported. The waters recede by up to half a mile at low tide, making it sometimes difficult to return to shore when the waters return.
Kateri Champagne Jourdain, a member of the National Assembly of Quebec, wrote on Twitter that she gave her full support to the mayor of the municipality, calling the deaths “a terrible drama.”
“The entire North Shore is shocked by this news!” she wrote.
It was not immediately clear how the group was swept up by the tide, but police said an investigation is ongoing to figure out what happened leading up to the event.
Erin Cunningham contributed to this report.