Here is the function graph. You may have to play with the colour map to get the (vertical) width of the dark strip correct for your requirements.

PS - I tried to keep everything to a minimum, just to avoid problems (eg shadows) distracting attention from the concept.

You can do exactly the same sort of thing if you're using a mixed material for your island texture - just insert the the "dist to obj. below" and colour map between the func for the texture that meets the water and the colour node.

Hope that helps....

Cheers,
Diolma