It looks good, but could use some work. The soft lighting around the legs looks like you just soft erased the turtle and place it right in.
I don't feel like the water is quite as foggy as it should be, and the rocks in the ocean floor look too big for it to be the size it should be, and should be smaller, since the turtle is a giant.
There should be a huge shadow under the turtle, since there is a relatively bright light source :P
Also, the turtle itself looks quite lq, as well as the ocean floor.
The water, where it is cut off, looks too flat and not very wavy at the line where it meets the underwater surface.
The effects look good, but could use more work on them, and maybe a bit more realism work added.
Maybe the piece could use some better depth too
It's pretty good, though, and I like it ^-^
Keep it up, man