@Chantal-Goetheer this is a tough one! I did a couple draw overs and really struggles with understanding the perspective. I went and looked up a couple of references and I think I understand better what’s happening. I think in order to pull off a cutaway image like this it would have to be much taller. I did a quick thumbnail so you can see what I mean. That option would require a complete re-draw though. something like this.
For the view you have, I don’t think a cutaway image works. I think you would need to change it so that you’re looking up at the crumbling cliff through the water…which would mean you couldn’t use your cool wave design (which I love!). Something like this.
These are the references I used to see this.
Working traditionally you can do a light blue or blue green watercolor wash over the underwater portion to give it the apprearance of being underwater. Underwater light is very blue.