The Undersea

Let me say first that this is a lovely idea, and your colors are very striking. Where it seems to fall behind, however, is in the composition, scale balance and color placement.
When using a two-color scheme, it's best to show dominance of one color over the other, and use the second color as an accent. You set out to do this, but got carried away in parts of the reef, particularly on the left. Combined with the large scale of the reef in comparison to the whale, and the top/bottom division of the composition, the color use feels too even-handed.
Regarding scale, your reef and your whale look like they're about the same size, which makes them both seem sort of diminished and claustrophobic.
All of these problems will be solved by a few simple composition/scale tweaks. Namely, scale down your reef to the lower third of the composition, and shrink the individual details to a quarter the size they are now. The right-hand section of the reef, in particular, should be MUCH smaller as it is much further in the distance. Remember, "scale checks" are provided by the little details, like textures or critters. Big only looks big when it's next to something small.

In all, beautiful work here. It just needs those finishing tweaks to be truly stunning. <img src="e.deviantart.net/emoticons/s/s…" width="15" height="15" alt="" title=" (Smile) - "/>