A manually propelled watercraft for transporting a user on water, comprising a buoyant body having a longitudinal axis and a top side and a bottom side disposed adjacent to the water, a track disposed on the body substantially parallel to the longitudinal axis, a vehicle having a bottom side and being slidably coupled to the track and movable to and from first and second positions relative to the body, and a fin coupled to the bottom side of the vehicle and being movable to and from an extended position that is inclined relative to the longitudinal axis and a retracted position that is substantially parallel to the longitudinal axis, wherein when the vehicle is moved from the first position to the second position, the first fin is substantially disposed in the extended position to maximize water resistance for propulsion of the body, and when the first vehicle is moved from the second position to the first position, the first fin is substantially disposed in the retracted position to minimize water resistance.