I would deal it off the best you can. I would fill in the banks with some kind of plaster. I used quickset drywall compound, once i had mine smoothed out, i let it dry shortly, then threw some paint on it. You will want to add some black to whatever area you want to seem like it has depth.
You can just paint the banks a earth like color... otherwise if you dont contain whichever water product you choose, it will seap through the foam and it will never build up.
I suggest you stay away from woodland scenics realistic water and instead pick up some invirotex lite at any home improvement store