Please post a picture of what you have. What type of "water" are you using and how deep do you intend to pour? I would suggest starting with a realistic-looking riverbed. Contour the river bottom with thin layer of plaster, paint multiple shades of brown, then glue down layers of real dirt, brown/dark brown ballast and talus of various sizes with matte medium or diluted yellow glue. You also might consider cutting a piece of basswood (1/16-inch thick) in the shape of the hull, gluing it to the riverbed where you want the boat (to give the hull a flat surface), and painting it to match the riverbed. Then place the boat in position when you do the pour. I have a "river" on my layout (without a boat). Based on my limited experience, this is how I would do it.