This implementation of caustics is physically fake. I do not represent light beams entering and refracting on the water surface. This effect is achieved by tiling a sequence of textures on the terrain for the area under the surface of the water.
This does require the graphics card to support a 3rd texture unit. If you remember from previous blog entries, the terrain already requires 2 texture units, 1 for the terrain texture and 1 for the detail map to add more realism to the scene. From some quick research it looks as though most modern graphics cards support at least 4 texture units so this should not be a problem.