Alright, I agree with placing the whole scene inside a cube of water, but in addition to that you should place a water plane directly infront of the camera. Also the spotlight on the submarine is totally unnatural, it needs to fall off on all the edges as well as project onto the ground. Also the sub could use a bit more detail mostly in respect to more interesting textures, because at the moment it sticks out really badly as a bright yellow blob. Add some rust or scum or something mainly. As for the windows on the sub they are just as bad, texture them if u can otherwise atleast make the material reflective or transparent so there is some sort of other detail on it. Also are you sure you AA'd the whole scene? The plants look highly aliased but that may just be caused by the fact that they are so different from the terrain? Again with the plant texture it needs more detail, I know you are going for the plastic look but even plastic has dirt/blemishes. The fish models are pretty good and the textures arent bad either, MAYBE make the mat a bit more specular (emphasis on the maybe, I dont know if this would actually help the scene) ? The main problem with the fish though is that they look like clipart pasted onto the scene, spread them out, they dont have to be too far back but put them at different depths where the lighting will intereact with them differently, this will give the entire scene more depth which it lacks at the moment. Another suggestion for lighting would be to use the softest shadows possible for this undersea scene, turn up the fog and have it fall off at a slower rate but have it start right in the very foreground of the image. You might also want to consider putting some colored spotlights in some of the darker areas and have the shine up on to the fish, be very careful with this effect though as it would be easy to overdo, just be very suble, it would be like the light coming from the surface reflecting off the rocks below, stick to greens and blues for this. Also I don't know what you use for postwork, but Paint Shop Pro has an amazing filter for lighting that comes with the program, I don't know if Photoshop has an equivelant filter but if you wish to know how to use it for your scenes I'd gladly show you, I use it for nearly everything I do, almost always helps dramatically. Also if you don't have it already, grab the freeware copy of Universe or download the plugin, it's filters would be very helpful for this image as you can sort of show the light shining down through the water and pillowing off the objects it hits most intensely; again if you need help with this technique I'd gladly teach you what I know, if you don't have Universe, Eye Candy 4000 has a good corona filter which may do as well. Anyway I think this image has a lot of potential and I really look forward to seeing revisions, keep us updated!