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.