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Don't know what to do next for this. I've pretty much reached the end of my photoshop know-how.
First of all, the cropping is very unfortunate. You cut of one leg right above the knee and half the foot on the other. It's looking like "I don't want to draw feet so I'll just pretend they're not there."
Moreover, the whole image is looking very flat. I attribute this mostly to the endless white void in which he's swimming, but also the artificial lighting which does a great job making him look like a plastic figurine, or a paper cutout in some places (the one knee he has).
All the lighting and shading is too blurry. Work in deep shadows with hard brushes and highlights with the same.