We are currently in the process of overhauling our material system, which should be ready soon. Once that is complete, we will create a tutorial on custom materials.
Currently, you’ll want to make sure that the Water Material parameter on Cesium World Terrain is set to M_CesiumOverlayWater. This parameter can be found in the details panel, under the Cesium | Rendering category. Also, try hitting the “Refresh Tileset” button. To make a custom water material, you’ll want to duplicate M_CesiumOverlayWater and do your edits there for now. This material samples the watermask to determine where to place the water material, and so your setup will have to be similar in order for the watermask to work.
If you still aren’t seeing the watermask, please try checking in a different location, and then let me know if you still aren’t seeing anything. The watermask currently does not work on all bodies of water. Most large bodies of water, such as the oceans, should work fine, but others (like the Great Lakes) currently do not have a watermask.