The water is a volumetric slab. Now I increased the diamonds' ambient and refraction as Hawkfyr suggested, and changed material to the plants and it seems to look better. But still a little bit unreal....I added some corals created sith lattice. I would not like to darken too much the scene, as I intended to create a caribbean underwater scene. By the way, I'm also working on another picture, and noticed that when i change the material to one object (i.e. a flat square)all othe object of the same type change material.....maybe I clicked something I ought not.....sigh......