The boat's engine drives it through the water, but the water itself is flowing. What the riverbank sees is the sum of the two: the boat's velocity relative to the water plus the water's velocity relative to the ground. Steer the boat and watch how the current reshapes its path.
Aim for the dot: the hollow marker on the far bank is directly opposite the start. Angle the boat upstream until the amber resultant points straight at it — that happens when sin θ = v_r / v_b, and the drift falls to zero.