I've got this much sewn together (after lots of improvising). The bit of lower sky is just pinned, and the way I will quilt it will give it some distinction from those far-away islands. I also have the rest of the sky ready to piece and put on the upper half.
Next, a jungle of leaves go around the bottom and lower sides, with palm trees up to the top. This one is a king-size and taking lots of hours. Also the driftwood is just pinned on because I'm not sure where it will go, and I might resize it.
Here is my sketch for the leaves and trees, but as you can see, it is only a guide. I'm hoping the leaves will be a lot easier to make that that water and sand!