I think that the light and the subject are good. But IMO, the main subject (nudi) is too small in the frame - especially when viewed at this size. It is also placed dead centre, which emphasises how small it is and does not invite the eye to explore the rest of the frame.
The image would be more dynamic if the boundary between the kelp and the blue water ran on a diagonal, rather than straight up and down.
The strengths of this image are the nudibranch and the interesting light cutting across the kelp, creating areas of highlight and shadow around an interesting subject. Give the viewer more of this good stuff, with a tighter crop, to they fill more of the frame.
The background is probably 1/2 a stop too dark, which would enliven the image (drop the shutter speed to 1/90th or 1/60th). It would also benefit from a bit more interest (some surface texture, light beams etc) to add a bit more depth the image. Solid colour backgrounds tend to leave images looking a bit flat.
Anyway those of some of things I would have thought about - although I am not sure how much thinking I would have managed at 3.5˚C!
Alex