First we must assign a material to the ground. Select ground from the list of image components in layer 1, on the right of the workspace (outlined in red in the example). Now click the button for assigning a new material to the ground (outlined in green). From the list of available collections in the material dialogue box, choose 'rocks'. There are several suitable choices for the sea bed. On the top row of the rocks samples are sand, wet sand, and river bed. Of these, I choose river bed, but the stones are too big and too numerous, although the wavelets in the sand are nice. So now we must modify the material a little. Select the river bed material and OK the material selector box. The river bed material will now appear as the ground material at the top right of the workspace. Double click on the sample to open the material editor. Change the value beside the material 1/material 2 slider to 48%, and the scale below Material 2 to 0.4. OK the material editor, and in the main workspace, change the scale below the ground material to 0.4.