How do I make the waters edge visible under water?

I have a fair amount of experience using Unity, but am yet to dabble with water.

I am using Unity Indie(? non-Pro), and notice that when my character goes under the water level, the water disappears, there's no "fog" and the edge where water meets the terrain is gone.

I am briefly aware that I need to trigger fog on submerge, which I'm sure I can figure out, but how do I have the "line" visible - is it a matter of merely having a second water mesh "upside down" at the same place?

Thanks