Well I got idea on how to make walls here. For that I needed for filter not to work in some parts of picture. And I could define rectangles on which filter works. So I wrote and algorithm that breaks rectangle on to 4 peaces around provided rectangle. With that I was able to add something like walls here. Form more info go to previous version of this toy: https://wonderwhy-er.deviantart.com/art/Bitmap-Echoes-74820873

Controls:

Mouse creates waves where you point the mouse.
Click and hold mouse button to draw a rectangle. Release mouse button to apply the wall.
Use clear to clear all walls, use random map to add random waves to the map.

Edited
Ok i changed formulas to different variant. I diferances of water level with diferrent trick now... Spent two hours on that :( Now i am tired. But at least it works better now. BTW i used this http://www.gamedev.net/reference/articles/article915.asp
I was thinking on adding edge effect but i think i will not. Need to attend to other things now...

Changes:
Now not point but line is drawn under the mouse.
Number fields now mean different things. First one for line width. And second one for line wave height.