All things come with time, training, and practice.
Couldn’t be said better! Here’s more what I had in mind for the black depths:

Not e how the darkest part of the water is a deep navy blue but still shy of black. I still think this looks like deep water but in a more appropriate spectrum as the light shafts and specular level of the shark’s body.
Values shy of white and black also leave room to adjust in compositing. Once values are committed to white or black it’s hard to do much with them.
Hopefully there’s a nugget to be gleaned from this. Good work overall 👍👍