Was working on this:


Bonus top-down RPG sprite (Note head-fins' drastic effects on silhouette, even small scale)
since earlier on in the thread. Design has progressed pretty well since but it can definitely be pushed further. Most problems have been pointed out by others in the thread however.
All these good refs and no one mentions the
Zora from Zelda:OoT/MM (specifically Link's form, or that one with the fins)?! The fins here don't have to be as large as the fins there, not even in the same place, but should have more of an impact on the character's silhouette is what I'm saying.
Zora as an example, proficient swimmers that are aided by their arm-fins. As a result, said fins are pretty big, almost wing-like. Here, I ditched the shoes, figuring this guy swims with heavy use of his feet (but ankle-brace is still there, you see the symbol on it repeated a few times hopefully unifying the design.)
The palette/colorscheme (light blue to dark green) chosen here does bring up some interesting hue shifting and color conservation (10 colors). Also rimlighting, because rimlights are awesome, and in this instance, show the transparency of fish-guy's fins. So that's really a bonus, but you don't have to do this to demonstrate transparency if they're clearly fins. Both of these come down to a matter of choice.
With the face you have it just seems like a demon anime guy with aquatic features that could be more prominent, here it's more a cross between a primate and a dolphin/shark...with tusks...or something like that. Bit odd in retrospect but, point is, the face in its general form can afford to be a bit less humanoid.
With what I did here, defining w/shading more than line, I pretty much took a completely different rendering direction than what you were going for but this might give you ideas on design, lighting, correcting anatomy etc. Btw I would recommend straight cel-shading for your style, starting again from two contrasting shades (either in the light or it isn't) + lines and working a bit from there, not over-rendering.
Plan to rough out the desert outfit later, so keep an eye out for that, I'll be back for a while.