Water has a different refraction index than air (n =1.33) and because of this it causes the light ray to refract, i.e. change direction. Our eye follow this broken light path, but our brain forms an image as if it was situated on the continuation of the straight beam. Due to this, some objects that are normally closed by the vessel edge, can become visible through an optically thicker medium, such as water.
To visualize this, refer to the picture at http://www.wisedude.com/images/coin.jpg