The water mask on Cesium World Terrain doesn’t cover all water bodies on the planet. Fortunately, due to recent material changes you can define custom areas of water with a Cesium Cartographic Polygon.
You ask at a convenient time, I’m currently working on an addition to the Cesium for Unreal Samples that includes a material that does this automatically. If you don’t want to make your own material, you can download this premade material by cloning this branch of the Cesium for Unreal Samples repository. You’ll want to copy the MI_CesiumCustomWater material and all of it’s dependencies in to your own project folder.
Then, add a Cesium Polygon Raster Overlay to Cesium World Terrain. Set the layer key to CustomWater, and uncheck “exclude tiles inside”. Add the material on to Cesium World Terrain in the Cesium|Rendering category of the details panel.
The Cesium for Unreal Samples update should be published on the Unreal Marketplace around the start of the new year. I’m also working on a tutorial to cover all of this more in-depth.
Let me know if you have further questions.