Provided the prop is not too big, with a displacement hull, it becomes self regulating. A propeller in water is effectively a torque converter. Just get one a bit smaller than the outside diameter of the motor, the motor spins the prop, the prop moves water backwards, the hull goes forwards. To move the hull forward its own length, the prop has to move the same volume backwards, plus some for luck. it doesn't really matter whether its a wide slow column of water that is moved, or a narrow fast stream, apart from the fact that the boat going forward cannot go faster than the water going backwards.
Any river means moving water, you do need speed to overcome this. As long as its not a silly racing motor, direct drive is fine and has the advantage of simplicity.
RC gear was originally designed to run on 6 volts as this was convenient for either 4 dry cells or 3 lead acid cells.