I think it looks great so far but I would adjust your specular shading on the dolphins foreheads. It wouldn't be that defined while underwater. If the dolphins surfaced, it would, though.
A couple of other things to think about: the ocean itself is a deep deep blue which for the most part tends to mean deeper water (the deeper you go, the less light there is...reds get filtered out first, etc.) so I'm wondering about a couple of things:
1) Would the reflections off the bottom and the dolphins be so strong at that depth? I think you'd only get that if they were swimming closer to the surface. Look for some pics around the net and see.
2). There is no real sunlight definition in the scene causing the reflections.
3). With just a sandy bottom and no terrain, it seems like it should be shallower.
These are just some things that came to mind...I think the scene looks great, though and I think anyone who isn't nit-picking is going to love it. Good job!
"Terminat Bora Diem, Terminal Auctor opus."