What is meant by No Work Done?

In physics remember that no work is done until and unless body doesn't displaced under the action of the force. Consider a man is trying to drive his boat by rowing upstream. If the force exerted by the man is equal and opposite to the force exerted by the flow of the river, the boat will not be displaced relative to a point on the bank.

So, although the man is applying a force , the work done by this force is zero with  respect to an observer on the bank. However, with respect to the flowing river, the man is doing work, as the displacement is not zero then. This displacement is given by the distance by which the man is carried downstream if he had not rowed. The work done is zero when the force is zero or the displacement is zero, or the force and the displacement are at right angles to each other.

Why cos theta is used in the formula of work done?