Lame…but ill help. Layer new-solid-grab the pen tool ( and click on rotobezier ) turn off the solid but have it selected and draw a mask around the sand) ..choose color for your solid or ad a fill or a gradient ramp. this particualr example is just a blue solid on normal, however if he had added a tint effect to a duplicate layer of the sand and animated the mask, it would have looked nicer, But its just a solid with 50% opacity or so…Now you have your blue shape, hit MM and start animating the mask path over TIME