While still in the Vertex sub-object mode, go to down of the Modify panel and expand "Soft Selection" roll-out. Then Check "Use Soft Selection" and increse the value of Falloff.
Now, maximize the Perspective viewport and select some vertices and move up/down them to have an irregular shape of the land. You may still have to adjust the Falloff value in Soft Selection roll-out.
Choose Create Panel > Geometry > Standard Primitives > Box. Then create a Box object in the Top viewport. It will be our Water surface.
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