Notice how the reflections of the family in the sand guide the viewers eye to the family. These compositional lines are referred to as line of repetition & design. Another nice element in the design of this image is that the horizon is in the upper third of the frame. THe result is that the heads of the people are within the water and the image has a smooth flow. Many times photographers put the horizon in the middle of the frame and the result is the ocean cuts through the heads of the subject and this interrupts the flow.
More tips on composition can be found among the blogs here.