I've never done one either but I have a few ideas for you.
See if you can find a color sheet with a storm at sea pattern and color it in the way you want. Sounds simple enough but it might not be.
The other thing would be to start collecting blue fabrics from very light (almost white) to very dark (midnight blue). You may not need much of some and it could be the color you need is not even from a "blue" fabric but maybe a flower or a piece of a design from another fabric color family.
Good luck and keep us posted with your progress.
kathyd