Just follow these steps!
First make a photo with an old tshirt that you have torn open like this
That make a second photo with the same pose, but without the tshirt.
Make sure you are on exactly the same location as the picture before.
Now take the part of the photo WITH the shirt and erase all unnecessary stuff (with masking). The shirt will not match exactly so you will have to adjust the size and maybe rotate the shirt a bit so it fits.
Desaturate the shirt when it is in the right place and up the levels to create a very high contrast.
Set the white layer of highlights to Overlay, you might need to lower the opacity of the layer.
Add some nice shading to the part of the skin to make it more realistic. Take a look at the original photo to see where the shadow is located. (TIP: you could use the original shadow)
All your image needs is for you to add a vignette, a little photofilter and sharpen the entire image a bit.
That’s it, your a fish!