Port Alberni RCMP say they launched their police boat around 4:30 p.m. PT on Saturday in response to reports of an on-the-water incident on Sproat Lake in the Alberni Valley.
Quick-acting bystanders were able to rescue one man from the lake.
He was later airlifted to hospital in Victoria.
By the time emergency crews arrived, police say the boat was fully submerged.
A police dive team is searching for a second, 41-year-old man who was onboard the boat at the time of the accident.
Police say they don't yet know how or why the boat sank but that an investigation is underway.
Google Map: Sproat Lake