A pedal driven aquatic propulsion system comprises a driven sprocket located substantially vertically beneath a drive sprocket. The planes of rotation of the drive and driven sprockets are disposed substantially at right angles to one another. An endless chain drive transmits power from the drive sprocket to the driven sprocket, operating in a twisting three-dimensional orientation as it loops around the sprockets. A pivoting, self tensioning idler arm is adjustably mounted on a frame that encloses the propulsion drive, and comprises an idler sprocket disposed in rotatable contact with the chain drive to maintain the chain under constant tension. The plane of rotation of the idler sprocket is tilted and offset from the planes of rotation of the drive and driven sprockets. The constantly tensioning system comprises a series of stationary and flying idlers aligned to the natural twist of the chain.