A boat propulsion system interposed between a prime mover and a propeller shaft comprises a hydro-mechanical transmission including a hydrostatic transmission part and a gear transmission part. The hydrostatic transmission part includes a variable displacement hydraulic pump having a movable swash plate and a hydraulic motor fluidly connected to the hydraulic pump. The hydraulic pump is driven by the output rotary force of the prime mover. The gear transmission part is drivingly interposed between the hydrostatic transmission part and the propeller shaft so that the gear transmission part is driven by the input rotary force of the hydraulic pump with the assistance of the output rotary force of the hydraulic motor. A hydraulic clutch system may be interposed between the gear transmission part of the hydro-mechanical transmission and the propeller shaft so as to select one of opposite output rotary directions of the propeller shaft.