Extra details and minor adjustments
I created a sense of cohesion with textures and gradients, but these also lessen the sense of depth and contrast.
To finish up my illustration, I create an “extra details” layer under the textures, and add details such as reflections and shadows to improve the sense of depth.
Once I added the reflections and other details, the whole illustration came together.
I set the blending mode of the “extra details” layer, which I used for the reflections, to [Normal].
I used the [Airbrush] > [Droplet] brush to add some bubbles for an underwater effect.
I used a light blue, set the layer opacity to 60% and the blending mode to [Glow dodge].
 Overall tonal correction
When I looked at the illustration after adding finishing details, I felt that it looked a little dull.
I created a new tonal correction layer at the very top of the layer palette, and adjusted the overall color.
I selected the [Layer] menu > [New Correction Layer] > [Brightness/Contrast].
・ Brightness: 10
・ Contrast: 18
After adjusting the tones, the illustration is finished.