Move the new light away from the camera, and a little higher up so the light source itself does not show in the scene. Now we have realistic caustics on the sea bed, but to complete the undersea illusion, we need to add fog and haze, so once again click on the atmosphere editor button, and then on the sky tab. Set fog in sky thickness to 5% and maximum altitude to 25. Now click the Fog/Haze tab. Use these settings for a starting point, but you will have to change the fog colour to one that blends in with the colour of your sky and does not leave a distinct horizon line.