At the moment, our scene looks more like a sandy beach than the bed of the sea. The first step in the transformation is to add caustics - the reflections of moving light on the sea bed that come from the sun penetrating the water. You could get this effect with the sun, but I find the sun too uncontrollable for undersea scenes. You can delete the sun, but if you do, every time you use the atmosphere editor you get a message reminding you that you have no directional light, and offering to create one. I find it easier just to point the sun vertically upwards so it no longer affects the scene. Click on sunlight in the image component list, to select it. In the side view and front view windows, swivel the sun pointer, using the curved arrow cursor, till it looks like this. The sea bed in your sample render on the right will go black. We will put the scene's lights on layer 2, so select layer 2 in the component list. Now right click on the lights button at the left of the workspace, so that you get the flyout of the light types available. Select the left hand light type, and a light - a point light - will be added to your scene. Move the light till it is in the positions shown in the top and left view boxes.  At the top right of the workspace, where the material appeared when we had the ground selected, we now have the light editor. Right click on the top button at the left of the editor, and select no lens flare. The button colour will change from orange to blue. On the next page we will locate and select the light gel we need to use to get the caustic effect.