An improved boat lift with an hydraulically-powered jack is provided with a locking mechanism, not only for maintaining the lift in its raised position when water pressure in the jack cylinder is released, but also to make it difficult or impossible to unlock by a potential thief or vandal. In accordance with the present invention, this object is achieved by providing a pawl member that engages a recess in the piston shaft that can be removed only with water pressure applied through an attached hose. Access to the water supply should be restricted for purposes of security. The pawl member is attached by a pivoting flap to the end of the cylinder from which the piston emerges. The flap has a flat underside so that water pressure directed against it will easily dislodge it from a recess.