In keeping with the ocean theme, if you do a starfish motif in the center (or my favorite, a sea horse) & waves in the sashing, what about rows of continuous line sea shells in the outer colored square within a square. You could repeat it with a tiny seashell in each of the colored intersections in the sashing. I would then bring the motif from the square within a square out into the white triangles along the border, as a motif, and maybe use bubbles as your filler. And finish with more seashells in the border. Would look really cool if you do the starfish with trapunto/faux-trapunto.
I agree 100% with Leta that Judi Madsen is amazing with fillers. She's a great resource to figure out how you want to fill the space around your motifs.