What you are seeing in the picture above is a prime example of wave interference. Wave interference is, simply put, what happens when waves traveling in the same medium meet. There are two types of wave interference, constructive interference and destructive interference. Constructive interference occurs when two waves in the same medium tih displacement in the same direction (from the normal) meet. In constructive interference waves can have either a positive or negative displacement from the normal, as long as it is in the same direction. Interference of the Destructive variety occurs when two waves in the same medium that have opposite directions of displacement meet. In the picture above, you can see as the water leaves the Jacuzzi and enters the swimming pool the water makes a series of circular waves when it enters. The waves undergo interference and create the patters seen in the picture. There are both waves crests and troughs and at different point in small disturbances in the water where wave interference between them is occurring. The way the water moves into the pool is an exemplary example of another idea of physics as well. As you gaze down at the bottom of the pool you may realize that the water is casting shadows, but how can water cast shadows if it is transparent? The answer lies in the make up of water itself. Although water is clear it does not let all light pass through it, only 98% of incident light is transmitted through the water. The 2% decrease of light is the reason that the shadows appear.