When looking at tropical fish (for reference images for a different project) I found an image of a fish with a pattern with dark areas that looked like cut out sections and thought it would make a good base for a mech fish.

Left : My initial sketch, with sections and internal structures er sketched out

Left : The setup for creating the underwater environment

• Caustic mask (top line, is actually a plane)

• Area lamp (triangle shape)

• Water surface, another plane, but with a ocean modifier and mostly transparent material applied. The plane was also tilted so the camera could be more side on without the water edge becoming visible.

• Volume Water, cube, with principled volume shader applied

• Volume Particles. cube ‘duplicate’, with geo node setup to create particulates in the water

Side : Basic composite setup to add a glow/blur to the lights on the fish