This looks so great I honestly had to reeeeaaaly think hard on what to do to improve it. I think you should try and color the lines instead of keeping them black.
Also, Do you know what I mean if I say "values"?
Here is a video that explains it and can really take your coloring to the next level: https://www.youtube.com/watch?v=ax130yILbw0
Here's how this looks in black and white:
You can see here that everything here looks like it is evenly lit. as if light is coming from all directions and the light color is pure white. Like in a studio or sth. if we are under the sea then the light color should be blue and coming from above.
Here is how I would edit the values (bear in mind that I'm painting over this with a multiply and screen layer with my own style which means it's messy. if you apply this in your own style it would look better and more organized)
Light scatters in deep deep water so everything light softly and the shadows are soft as well. far things have less detail and contrast (which you already did perfectly) but I personally made them alot darker cuz there is so much water between them and our eye. so light scatters alot and little of it should reach our eye.
moving on to colors here's how yours looked like (sorry I had to separate both aspects to explain better).
Here's how I'd do the colors
of course my color editing skill isn't the best. so this isn't perfect,light could be bluer shadows could be cooler... it should be colored from the beginning with these things in mind but this is just to demonstrate.
when it comes to colors you need to chose one thing that you want us to look at more than the others. one thing should grab our attention and not multiple things fighting for our eyes. In real life things are not really the color they look like they are. you may think something is red but when you take a pic of it and sample the color it may actually be purple. so with weird lighting comes weird colors. don't be too scared to explore that.
TL;DR as you work remember the main light source, it's direction, hardness, and color.