An extendable and retractable lower deck for a watercraft is attached to and supported by one or more amidships hulls. The lower deck and hulls slide between an extended position and a stowed position on rails that are secured to the underside frame of the main, upper deck that is supported by abeam hulls. When fully extended, the extendable deck remains attached to the watercraft. A rotatable gate positioned within the lower hull can be used for improved accessibility to and from the water; the gate is stowed when its distal end is in the raised position and is deployed when its distal end extends below the bottom side of the hull into the water in the lowered position. Laterally expandable panels in the lower deck provide increased width, and other accessories used with the watercraft can be contained in one or more storage holds in the amidships hull.