1. Save your map, to many times have i done this and the WE has crashed because i forget to turn enforce water limits off.
2. Create an area of water where you want it to be, and make sure that you have enforce water limits OFF, you can access this through advanced options. (if you try this with it on then it will usually cause the editor to crash).
3. Raise the terrain back to its normal height.
4. Now lower the terrain until you see the water.
Do this in the area you created the water.