The pictures are of rowers on the Thames at Oxford last Sunday along with a view of the college boat houses. When you pull the oar, you start to push the water. The water is very heavy and has a lot of inertia so it doesn't move. By Newton's 3rd Law there is an equal and opposite force pushing back from the water on the oar. This is what pushes the boat. The boat accelerates as described by Newton's 2nd Law. Newton's 3rd Law was written as "To every action, there is an equal and opposite reaction" when I was at school. These days I teach it as "If object A exerts a force on object B, then object B exerts an equal and opposite force of the same type on object A". I think that the change is to emphasise that two objects are involved. The type of force involved here will be an electromagnetic contact force caused by pushing the atoms closer together so that the nuclei repel.
