A folding kayak includes fore and aft subassemblies which are joined together and are covered by a water-impermeable skin to provide a watercraft with the favorable attributes both hard shell and skin on frame kayaks, with none of the disadvantages. In addition, one or more hydraulic adjustment mechanisms provide the kayak with characteristics of a plurality of different rigid body kayaks, with different operational characteristics, suitable for open-ocean or closed-water environments as desired. Ribs which are part of the kayak assembly have a relatively flat profile, making the kayak perform more favorably in adverse water conditions, and also giving the kayak some resilience and flexibility when encountering obstructions in the water. In one aspect, the kayak subassemblies may be joined, at least in part, by corrugated splines. By rotating these corrugated splines within elongate tubes that form part of the respective subassemblies, it is possible to remove crystalline, corrosive, and other build-up. The kayak may be disassembled into a relatively compact, easily transportable package.