A boat propulsion unit minimizes a shock upon switching of an engagement state of a clutch mechanism. The boat propulsion unit includes an upper clutch mechanism that is arranged on an axis of a lower drive shaft and that can be switched between an engagement state (first engagement state) in which driving force of an engine is transmitted to a downstream side, and a half clutch state in which the driving force of the engine is reduced and then is transmitted; and an advance-reverse drive that is disposed on an axis of a front propeller drive shaft and a rear propeller drive shaft and that can be switched between a forward travel engagement state and a reverse travel engagement state (second disengagement state) in which the driving force of the engine is transmitted to a front propeller and a rear propeller in order to propel a boat, and a disengagement state (second disengagement state) in which the driving force of the engine is disengaged.