To provide a watercraft propulsion system of high efficiency in a turbine-type power-driven vessel, in which rotation of an active runner is improved to achieve high efficiency by forming a recess for housing the active runner in a shell of the vessel.

The system includes an engine 12, a transmission module 14, and an extension drive shaft 16 installed horizontal. The drive shaft 16 includes the active runner 24 synchronously rotatable with a tail end, and the recess 34 and a top wall 36 are formed on a water passage 33. A launching column plate 40 is installed on an opening part of the recess 34. A passive runner 52 not rotatable is disposed behind the active runner 24 and the stern 30. A nozzle 54 and a rudder 56 are installed behind the passive runner. In this unit arrangement, resistance is reduced, and formation of a position which is more likely to be damaged can be avoided.