I would say anatomy, lighting inconsistencies and a somewhat vague background.
Anatomically speaking, the definition of the abdominal belt is very graphic with black lines where soft creases are expected, which doesn’t really fit with the rest of your picture and feels more like scuplted plastic than actual flesh, plus they’re maybe a bit too wide. You’ve also shrouded the hip region in shadow when it should be in plain light, or at least receive some reflected light from the water, which removes consistency from the overall anatomy (because there’s no apparent reason to shade it so, the first thing that comes to mind is that you hid it because you’re not sure what’s actually happening in there)
Lighting has a good start, but at some point you seem to have lost track of the bluish secondary light that comes from the water. For instance, the blue highlights on the rear wall are so bright that I would hardly expect any actual shadow on the body of the creature. Then, there’s blue light on the closest rings of the foreground tentacles, but none on the pillars they’re wrapped around. Not to mention the reaching tentacle which doesn’t receive any light at all. If you’ve done so because he is stepping out of the water and there is no reflected light coming from the front, then make it more obvious. Show the edge of the basin, stone marches, or wathever.
Also, the water splash on the sides feel a bit grey. I would expect more specular highlights in them.
Next, the background could be more fleshed out. You went as far as to place fine textures on the foremost pillars, you’ve detailed the water splashes, but of what happens beyond these back pillars we do not see a thing. Merely a spot of blue that hints of a wall, but not even a cast shadow that would suggest how far it is from the pillars and how tentacles can find the space to wrap around them. Such a degree of simplification might be acceptable if there was empty space or a long distance to the next obstacle, but it doesn’t seem to be the case here.
Last, the picture is almost a duotone of greys and turquoise blue, which makes it somewhat dull. Adding a (not-so-) subtle contrasting colour somewhere might increase the interest of the entire picture. This one’s more subjective than the above, though.