You can easily create a swimming pool in OrthoGraph. You can draw it's shape as you would draw any other building or room. Select the location's property and set it's height to a very small number (eg.: 0.001 m). You should also set the floor covering to "Marine splash". You can add the depth of the pool as an additional property. Just click on the plus button on the the top right corner and add a property with the name "depth".
You can also measure the pool as any other locations in OrthoGraph. After you have measured it, you can check it in 3D View.
Here is a video about how to create a swimming pool: