Thank you all for your replies. I have considered all of the options mentioned and it has helped me to narrow down my choices. When I first thought of doing a scalloped border, I had thought that the scallops would mimic waves, and I think that might be the best option. I did do scallops on a different quilt (hopefully pictured below) and matched the binding to the border, which worked well for that quilt. I may do the same on this. Whatever I choose for binding, I think it needs to be one fabric. I think a scrappy binding would draw too much attention from the center. I will audition some bindings once I get to that point. Now I need to come up with a quilting design to fit within the scallops...I'm thinking seaweed and other sea creatures. I plan to quilt swirls in the water around the turtles.