The canoe moves forward, since when pushed backwards, the water exerts a forward force on the canoe in accordance with Newton's third law.
Newton's third law states that for every action, there is an equal and opposite reaction. Action and reaction act on different bodies.
When we paddle a canoe,we push the water backwards. This force is the action.
Water exerts an opposite force on the canoe, causing it to move forward. This is the reaction force.
The forward motion of the Canoe is therefore in accordance with Newton's third law.