An apparatus for a hydraulically locked accessory mounting device for attaching accessories to a boat, or vessel. The mounting device includes a base for attaching to the boat and a ball head that is attached to an accessory, which can be moved to a selected position and locked in that position by a hydraulic assembly. The hydraulic assembly includes a first piston moved by an operator. The first piston is in hydraulic communication with a second piston, which engages a shoe that locks the ball in position. In one embodiment, the hydraulic system includes a bleed valve connected to the channel between the first and second pistons. In one embodiment, the first piston is operated by a knob with a threaded shaft. In another embodiment, the first piston is operated by a cam on a shaft that is rotated with a lever or knob.