Chief Supt Dan Evans said: "We were called at 1.21pm to an incident at Roadford Lake near Okehampton.
"A boat carrying a number of people had capsized. Firefighters responded quickly and supported the rescue operation alongside other emergency services.
"Police-led searches have recommenced today, Thursday 9 June, and efforts continue to locate two missing people.
"Two people were yesterday removed from the water and taken to Derriford Hospital in Plymouth for treatment. One person has since been discharged and one person remains in hospital in a critical condition.
"Two other people were removed from the water and checked by ambulance teams before being discharged at the scene."
All of the six people involved in the boat accident are said to be from Devon and the emergency operation has not only involved the police and fire service but also ambulance, air ambulance, coastguard and search and rescue teams.
Roadford Lake, located in West Devon but close to the Cornwall border and a short distance from the A30, is one of the largest inland waterways in the South West and in places is 130ft deep.