If we jump out of a boat which is at rest, the boat moves:

According to Newton's third law, when two bodies contact, they apply forces to each other that are equal in magnitude and opposing in direction. The third law is also known as the action-reaction law.

According to Newton's third rule, "Every action has an equal and opposite reaction," therefore if we jump forward, it exerts a force on a boat in the opposite direction. As a result, the boat goes backward.