It's very easy! After you create your map, select water from the terrain/ground dropdown at the bottom (the water in the sidebar won't work for some reason), choose one of the smaller water tiles (selections 9–13), hold Ctrl and left click.
Anyway, I thank you so very much for this tutorial, RaVaGe. I was pulling my hair out with the cliff painting tool, and I had no idea about using the framework mode to make cliffs much easier to create. I just finished creating my first-ever map, and I could not have done so without this guide showing me that making cliffs is easy and actually kind of fun. Cheers!