Hi! Sorry it took me some time to get to this

First of all, I like that you're trying to put some character and expression into the character's face. It's not just a standard smile, making it a more interesting emotion. The anatomy isn't terrible, but could use some work still, though, I think you did a pretty good job on referencing the anatomy of the shark. The pose is definitely a little too stiff. There is a nice sense of color here, and I like the blue hues you suggested, but value-wise, the piece is a little flat. The background is underwater, and therefore harder to make interesting, but I think you could have pushed the atmospheric perspective you already have going on and made a few foreground elements darker or more prominent in order to add more depth to the image. Overall, I think it's a pretty nice painting/character study. It looks fun and I really like the hair reacting to the water. Your treatment of shading seems pretty nice and you seem to have a good sense of how light works. You can just push everything a little further.

Now to go into more detail.
First of all, I think the thing that is lacking the most in this image is a dynamic pose. The character here is facing us, looking at us, is very perfectly framed as to prevent any cropping, and looks like he's just chilling with hardly any movement. I'm pretty sure this is because you said the piece started out as just a facial study, so you probably weren't worrying about the pose as much. Even so, the best way to give a piece more visual interest is to have an active pose. Before you start painting, try a series of pose sketches and try and find what's the most interesting. Don't worry if you crop some of the character. Or, in this case since he's looking at the viewer, think more of how he can hold his arms or his tail in order to appear more dynamic.
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Here is a super fast sketch I just did. Just with a few simple changes such as a more dynamic arm and hand motion, as well as swishing the fin to add movement (bonus points for overlap) the pose already feels far more dynamic despite the changes being relatively simple.
Also style-wise your gallery seems to be going for a semi realistic yet still cartoonish approach to expressions. Play with pushing the expression anyway to see how far you can push it. and then see what other elements of the character you can pull back in to re-enforce the expression chosen. Scrunch up the whole face and tighten the hands and tense the muscles for anger, or loosen everything up for happiness and laughter. Let everything droop with sadness.

As far as values go, if you grayscaled the image, there's not much of a difference between light and darkness!
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Allos yourself ot push the lights and darks as much as you can! Since the piece is underwater, you can also have fun with the lighting. Water really captures rays of light as they go through it, and it also tends to silhouette people against the light.
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Shadows feel heavier and bluer, and the light gets very warm. Also, play with the way the sunlight ripples across figures underwater. You can cheat this effect using dodge if you do it carefully.
Also, things in water tend to fade to blue very fast, so if any elements of your character are further away from the camera (such as the hair) you can start to let it fade into the ocean.
[link] (super rough paintover to show what I mean. The colors are wrong since I just played with the levels)

Another thing that could make the piece more dynamic is the background. Right now there are bubbles and some sea weed, but there is hardly any overlap and the bubbles don't re-enforce the movement of the character. Allow some of the background to obscure the character, and try and place bubbles wherever he would be moving. Any way you can re-enforce the movement of the character is a good idea. Additionally, I made the darker just a little darker and the lighter a little lighter here.
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Now, I won't pretend that is the perfect piece, since your background had some good composition, but instead, it's just a visual guide to what I was explaining.

The biggest thing I can recommend to you, I believe, is just to keep PUSHING. Try a few separate sketches to see if you get a better idea before you work to finish. Or, before starting color, try some very quick color keys. While painting, turn your image grayscale to see if it still reads!

Anyway, it's a good piece overall, and just keep working hard
I'll get to the other piece a little later.

Wow I love it This is so cool. :3

I love it. But is it just me, or is his middle a little chunky? Then again, if he were skinnier, I'm not sure it would look right...