Newman Atkinson says:

Barbara,
Depending on what you are trying to show. If you are just holding the thin layer from dripping off to the floor I have used masking tape or even tack up some hardboard to dam up the water product you wish to use. Most cases the water product is not put on deep at all. it is usually a plywood or a hardboard of some kind and the deeper the water the darker you paint the board to a sandy color along the shore area. to show shallow. A couple of thin coats of the material you apply will look really great. Prior to the water product a few stones or a stump sticking out of the water should be mounted to also include driftwood and water plants growing out of what will be your water. Don’t forget the fisherman in his waders You will want to place him at a level that he might be at and trim off the feet or legs to give him a depth look. Don’t place him in deep if he is in the shallow sandy color area (you might wish to move him to a level toward the darker water to give the affect of depth.
Some like to show the water from the front edge of the layout as if you are looking at something under water. Sometimes tapering the depth down near the edge of the layout will help to help show an object from the side such as a submerged old car. The item could still stick out of the water some. In this case the water could be dammed up at the edge by using a metal smooth piece clamped up to the side of the layout face board. As it drys and can support itself remove the Dam so it will dry without locking to the dam. Do the same on subsequent coatings As that part dries additional coats to cover the rest of the water scene can be added without depth and the color under the water will do how the depth will look. If ripples or moving water is needed the later coats can be swirled and there are products to give it some white water effect if you wish. Although I have only done a few water scenes they are not hard to do. Things like bridge piers and posts for docks need to be in place before the water is added as you don’t want to make the bridge post look like it is sitting on top of the water. Don’t forget the boat you wish to float. If you are needing it to show the under part of the boat you will want it near the front where the depth of your water is where you will see the hull. Or you may sand off the bottom depending on the model or if you can’s sand it off then inset it in the plywood You will need to do most things prior to adding the water even if you are only doing a thin layer. hope this helps, from Newman