maybe you could arrange halves or quarters of drunkards path, done in varying shades of blues, to look like rolling waves? so it comes out like this, sort of: '' only of course more pronounced with the half/quarter circles....
If you are adding sea life you could do a star block or two, with blue/water background and orange/yellow/red (one or the other) for a sea star....
anything with gently curved lines (a gentle curved rail fence block, for ex) instead of straight would work as well....
Sounds like a delightful quilt! I wish you luck with it!