A double-effect hydraulic actuating cylinder, particularly for hydraulic steering devices and for marine outboard motors, includes a cylinder slidably fitted on a coaxial rod, or vice versa, which sealingly projects out a head of the cylinder and carries a separating piston dividing the cylinder into two variable volume chambers, each connected to one of two inlets/outlets of the pump and a sealing head for the mutual sliding of cylinder and rod. The rod is non-slidably connected to a bracket fastening the cylinder to the motor, and an idler arm is secured to the motor and integral with the cylinder. At least one of the sealing heads include a sealing element engaging the inner side walls of one of the two variable-volume chambers, and a separate, coaxial plug element placed at the end parts of the cylinder and preventing the sealing element from sliding.