I was admiring how even with mostly just lines and very little in value work at this stage, the turtle is reading as metallic. I can't wait to see this painted.
Value-wise, I'm reading the upper part of the illustration as mid tones, the bottom darks and the turtle and girl as light. Is that right? If so, I think that works well composition-wise, and there's lots of great potential for the lighting.
I think you've set up a great visual-flow. My eye immediately went to the girl, then I wanted to see where she was looking, saw the people above, came back to the girl, noticed her hand and then noticed the turtle. You might be able to push the shape/gesture of the swimmers above her to really get the viewer's eye to come back to the girl.
The point of view/camera angle is also working well to me. Having the turtle askew like that really adds to the under water feeling. I notice he's swimming right out of the picture. Is that part of the story telling (I don't know the prompt)? If not, you might want to shift him/add some seaweed or something to help contain the scene.
I really like where this is going. Can't wait to see the next phase.