A boat crosses a river that is flowing to the right. The boat's speed and heading are set relative to the water; the water carries it downstream at the same time. Where it lands and how long it takes both come from adding two velocities. Aim straight across and you cross fastest; aim upstream and you land closer to the point opposite, but the crossing takes longer.
boat relative to water current boat over ground predicted path
What to try. With the boat faster than the current, press "least drift": it lands straight across, at the heading sin θ = c/vb; compare its crossing time with the fastest crossing. Now raise the current above the boat's speed. No heading can land straight across any more, and "least drift" switches to the best you can do, sin θ = vb/c; the note beside the buttons tells you which case you are in. Tick "view from the water": the boat's path becomes a straight line along its heading, and it is the banks that move.
Units: meters and seconds. The current is uniform across the river unless "fastest mid-river" is ticked, which uses a parabolic profile that is zero at the banks and c at the center (mean ⅔c); in that case the path curves and the water-frame view is not available, since different parts of the river move at different speeds.