What you're looking at there is an engine powered by a 2.6 knot water flow. Nothing else. The motion in the pistons is induced by the generation and shedding of vorteces in the water due to cylinders placed perpendicular to the flow.
The power density of the system is only slightly lower than that of a coal plant, and a farm of these structures placed in an analogous environment performs with nearly 15,000 times the power density.
Unlike underwater turbines, these structures are passive and safe for marine wildlife. They are also highly scalable, since you are not required to use a minimum length/width/etc. To achieve oscillation, you need only get within a 60% harmonic environment, which leaves HUGE tolerances in the components.