First make a photo with an old tshirt that you have torn open.
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.
Change the blending option of the layer of the shirt to Screen mode (keep opacity at 100%)
Now the magic will start to happen, add a nice looking fish/shark to the mix. The kind of fish is not important at all, just look at the position of the fin compared to your photo. It should fit...
First mask the area where you want to change the skin type. Duplicate the skin a few times to cover the whole area.
Add the fin to the image, use the mask of the first skin layer, but cut out the part where it will stick out.
The skintone of my shark was a bit brownish, change the color with a nice photofilter to enhance the image.
Highlite the lighting on the top of the fin with a soft brush in 50 % opacity.
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.