A boat propulsion unit has a construction that prevents a structure near a drive shaft from becoming complicated and increased in size. The boat propulsion unit preferably in the form of an outboard motor includes a front propeller that is rotated together with a front propeller drive shaft during both forward travel and reverse travel of the boat, a rear propeller that is rotated together with a rear propeller drive shaft in an opposite direction of the front propeller during both of the forward travel and the reverse travel of the boat, and a forward-reverse drive that is arranged on an axis of the front propeller drive shaft and the rear propeller drive shaft, and that can be switched between a direction in which the front propeller drive shaft and the rear propeller drive shaft are rotated during the forward travel of the boat and a direction in which the front propeller drive shaft and the rear propeller drive shaft are rotated during the reverse travel of the boat.