You can make still or flowing water features in any model scale using simple techniques based on dry florist's foam and clear sheet styrene or acrylic. There is no need to use noxious chemicals to create the water effect. Acrylic Artist's Mediums are used to create a textured 'water' surface on top of sheet styrene or acrylic, available from model shops, online or from some framing shops.
The basic technique involves creating the underwater scene on shaped dry floral foam, sections of which can be glued together before shaping if you need a long feature for a model railroad layout. Once the underwater scene is created, the area is 'landscaped' into the floral foam, and a sheet of textured clear plastic or styrene is laid on top of the water feature glued into position, and landscaped to disguise the edges.
Take a look at all the steps for how this dollhouse scale koi pool was created, then plan your own still or running water features, in any size or scale. Just make sure you use clear sheet plastic or styrene that is thick enough it will not bend in the middle when laid over your chosen water feature.
This technique is especially useful for simple water features like scale swimming pools and water hazards for model horse jump scenes.