Hi im assuming your working from a pattern,so this suggestion may not be what you would want to do.but,remember the ocean isnt just blue, its also shades of green,and white(top of waves) it also has black init.
I dont know who makes this fabric or what its called, but have and seen, fabric that is solid in color, going from lite to dark. you could use fabric that has ocean scences,or water/river panels. also you need seaweed/keip.
you could at the bottom wigglely lines of colorful fabric,pointing upwards representing the ocean floor and what you see when you sucba dive. Or maybe not Dave