Abstract: A boat lift for raising and lowering a boat in a boat slip disposed between spaced apart parallel dock sides, the lift having a pair of cross members disposed perpendicularly to the dock sides and supporting a boat support frame. The cross members are slidably connected to parallel guide arms angularly attachable to the dock sides via adjustable dock clamps. One or more tanks are attached to the cross members for supporting same at selected heights in a body of water, and pressure hoses and valves are provided for selectively admitting compressed air or water into the tanks for raising and lowering the boat support frame relative to the water.