Glue blue construction paper inside the back, sides and bottom of a cardboard box to create an ocean water scene. Leave about three inches at the top of the inside of the box to draw in a sky, clouds and birds flying. Use fishing line to hang fish cut-outs from the top of the inside of the box. This will make the fish look like they are floating in the water. Add pieces of coral purchased at a fish supply store and pebbles to the bottom of the inside of the box. Attach with hot glue. You can also add small plastic crabs or lobster to the bottom of the box. This will create an underwater scene.
Create a lake in the middle of the bottom of your diorama box using blue modeling clay. Make the lake large enough to cover a good portion of the bottom of the inside of the box. The blue modeling clay will stick directly to the bottom of the box without drying or becoming hard. Shape the surface of the lake with small waves or put ripples in the clay to give the appearance of movement.
Cover a board with a piece of 3-inch thick syrofoam. Carve out a river bed in the middle of the styrofoam and cover the entire board with pieces of paper towel soaked in Plaster of Paris. Let dry overnight. Paint the scene with blue paint in the river bed and green and brown paint over the rest of the scene. Add texture to the green and brown areas by using moss, rocks or turf texture paint available at hobby shops. Add other scenery appropriate to your river. Create the water in the river with a product called Easy Water. It pours on like glue and dries to a shiny, watery look over the paint. This product is available at hobby supply stores.
Draw an outline on your diorama in the shape of a stream. Paint inside the outlines using blue, green, gray and other colors to make a natural looking water feature. Give the painted water feature time to dry. Overnight is best since you will add a clear coat to the paint and you don't want the colors to bleed. Cover the stream with several coats of high gloss polyurethane and let dry between coats. This will give a wet look for your water.