Consider the movement of a fish through the water. A fish uses its fins to push water backwards. So then, the water responds by pushing the fish forwards, moving the fish through the water. The amount of force that was on the water is the same size as the force being put on the fish. The water is being pushed backwards which is the opposite of the fish which is going fowards.
For every action, there is an equal (in size) and opposite (in direction) reaction force. Action-reaction force pairs make it possible for fish to swim.
