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.
Add a new layer- right click in the Scene Panel and choose . We can call it e.g. local_water_shape.
Select the new layer.
Go to Tools and Terrain Edit Tools (Shape Brush → Flatten).
With the tool active, go to the Editor main screen and experiment with “flattening” the ground (basically, making a hole in the ground).
Now right on the area you have just flattened and choose Graphics → Add Local Water Areas.
Use the Scale and Move button on the left the change the size of your water area and bring it in view.