Learn how to edit the terrain and add water areas.
Adding water can be done by using Local Water Areas - areas are treated as boundaries of the water shape that is applied usually inside river or lake beds, where the terrain goes under a specified elevation level.
First add a layer (e.g. local_water_shape).
Then edit that layer with the Terrain Edit Tools (Shape Brush → Flatten).
Now right click, Graphics → Add Local Water Areas.
See the video for more details:
When you open up the World settings tab at the right side of the editor, you can search for a setting called 'disableWaterShaders'.
This setting should be switched off to make the water defined by local water areas visible.
Also please make sure that when you open up the environment Editor, leave the 'Allow water shader' option checked:
You can open the World Environment Editor using this icon button on the top toolbar:
