36-year-old Justin Harness, of Owensville, has been arrested for boating under the influence.
Osage County Sheriff Michael Dixon has confirmed a serious boating accident possibly involving alcohol on the Gasconade River at mile marker 28.
Authorities say the driver apparently drove off the river around 6:48 p.m. Saturday near Parson's Lane in the Mt. Sterling area.
His boat ended up in a tree.
The driver suffered minor injuries. His passenger suffered serious injuries, including a head injury, and was taken by Osage County ambulance to University Hospital.
Sheriff Dixon said he is in critical but stable condition.
Responding to the scene were the Osage County Sheriff's Department, Owensville Fire Department, Linn Fire Department and the Missouri Highway Patrol Water Division.
Authorities are conducting sobriety tests on the driver at the scene.