OK, so the sails are a different angle to previous pictures. They are both reaching, but in different directions. The floatation/buoy thing is visible breaking the (crude representation of) water surface. The counter weight at the bottom is not resting on the sea/estuary/river/lake/lough bed. In this design there is only one cable/rope/cord preventing the thing from floating downstream. Also shown is a simple squared rig, that sets the direction of the contraption, and is a fixing to the cable/rope/cord/etc.