I'd love to see the same kind of thing myself! Although I have no idea about rising water levels, I would like to see whether slowing can be added to other types of terrain. By using invisible splines, you can create negative cover wherever you want, but I am unsure about the slowing effect of water.
I'm currently working on a desert map in which I'd love to see something similar, where vehicles and infantry slow down as they ascent sand dunes, as their feet or tacks slip deeper into the steep sand.