Interesting method you have there with the cowhides. I use CorelDraw/PP 15.

When doing cleanup on images that are to have a transparent background, what I normally would do is select the magic-wand selector tool and raise the tolarance a bit like 20 - 30. Click in the white area, invert selection, and cut. Refill layer with white solid. Paste the copied image of your island and use a feather of 3 - 5. Less given sharper lines. Then use the magic-wand select tool again with a tolarance of 40 - 50. More if needed till it avoids selecting the small feathered dots. Then just repeat the cut, fill layer with solid white, and paste. Eventually you'll have a clear coastline. Also don't forget to invert selection to get the island.