Question

To take the boat away from the bank of a river, the boatman pushes the bank with an oar. Why ?

Answer 1

While rowing the boat, the  boatman pushes the water backwards with the oar so that the water exerts an equal and opposite push on the boat which makes the boat move forward. In fact, harder the boatman pushes back the water with oars, greater is the force exerted by the water and faster the boat moves forward.