This method of passing a boat from one shore to the other is seen on the La Nga River in Vietnam. No motor, no human power.
Blue arrow shows the flow direction. Because the boat is set un-perpendicular to the stream, there is always a force portion pushing the boat to either shore.
Yellow slider with roller can move along blue runway.
Black cable of constant length has one end fixed to the slider and the other to the boat.
Red cable has one end fixed to the slider and the other to a windlass on the boat. Thus length of the red cable is adjusted for changing boat angle in relation with the stream (in combination with the stream action), i. e. changing motion direction of the boat.