I had the same problems JVogel when I tried to use volume slabs for my water - everything was just too darn dark. I think the key is what ironbrew said you have to add lots of dim lights to bring out the detail of the objects but not too bright so that you still get that murky water effect. I ended up just using a water plane and playing with the sky. Someday I'll learn how to light things better and maybe I'll like using those volume slabs for water. Jade