As I was building the coastline for Magincia Island, I noticed that the tiles that comprised the coast were having a hard time refreshing themselves. When I scrolled the map in any direction, the coastal tiles would flicker between solid land, then one type of coast, and then another type.

It was at that point that I realized what the three different "layers" of tiles were used for. If you're running into similar problems, here's what I did:

First, I laid down the preliminary coast; just water and grassland.

Next, I clicked on the second layer in the upper, left-hand corner, and placed the western coast on top of the grassland tile. Now, this would be enough, if it weren't for the fact that this tile also borders the ocean on the south. So...

I clicked on the third layer, and placed the southern shore on top of both tiles.

The result, as you can see, is a rather nice blending of the grassland tiles and the surrounding ocean, in a nice shoreline effect.

Now, I'm sure that I'm the only one who didn't realize how to do this. But just in case, now you are ready to create wonderful shorelines of your own.