What is happening in the model? (extended Teaching Notes)
Surfers deliberately stay on the front slope of their wave (by swimming and choosing when to stand up), and angle their surfboards so that the rate of toward-the-shore motion matches the rate at which the water underneath them rises due to the wave action of the water. Since the rising water is in the part of its circular path that is moving toward the shore, surfers keep moving the direction they want even though each part of the water is moving back and forth. This strategy explains why surfers go forward but not backward, even though equal amounts of the water they are riding on go both ways (ignoring tides).
Because (in the absence of wind) a beach ball does not have any force acting on it, it stays on the front slope of a wave. The ball is also very light, so in the absence of wind it will tend to follow the motion of the water it is sitting on and thus move in a circular path rather than moving toward the shore. In practice, the movement of a beach ball will be determined by even a gentle wind, since it is very light, has a large area, and will quickly return to the surface even if it is briefly submerged by a breaking wave.
The actual water molecules in ocean waves move in paths that are usually nearly circular. In the shallow water close to shore, however, friction slows the water motion near the bottom so that the water near the surface moves faster, forming a "breaking" wave when the top gets ahead of the water that was supporting it.
What about crashing waves coming onto the shore?
Students may wonder "Why do waves break at the beach?" or ask about the difference between waves in the open ocean and those at the beach. They may note that the water of a crashing wave moves forward, not just up and down, as waves come ashore.
Waves break when the top of the wave is traveling faster than the bottom of the wave. As a wave moves towards a beach, the depth becomes shallower and the bottom of the wave slows down. Waves start to break when the depth of the water is just 30% more than the height of the wave. The top of the wave continues to move forward faster than the bottom of the wave, and the energy of the wave gets pushed up as well. A complicated combination of forces creates the curling lip of a wave.
Water does move a great deal at the shore, onto the beach and back again, but it does not continue moving forward. After crashing on the shore, the water that moved forward with the excess energy of the wave returns to the ocean or lake to come forward with the wave.
This level of understanding is not important for your students to have at this time, but some perceptive students may ask about waves.