A sailboat is moving at a constant velocity. Is work being done by a net external force acting on the boat?
This is a bit of a trick question it tricks you into thinking there is a net external force. The boat is moving at a constant velocity; that's a given. That means that the net external force on the boat must be zero.
But if I use Work = Force x displacement, even though the velocity is constant there is still some displacement done by the (constant) force acting on it.
Using work = force times displacement yields zero work because the net external force is zero.
You can decompose the work done on the boat into the work done by individual forces acting on the boat. Then you'll find that the boat's engine (or the paddler, or the wind) is doing positive work, but drag is doing negative work. They sum together to form zero. The net work done on the boat is still zero, no matter how you attack the problem.