Cesium for Unreal provides the option to toggle a water mask on the data you’re loading. However, the data must include this water mask data itself. Cesium World Terrain comes with this water mask, so you can enable it using the Enable Water Mask setting on the Cesium3DTileset.
Otherwise, if you’re using some other data, you may need to emulate this effect manually in a material. Maybe a naive start would be to apply the moving water effect to any areas that are blue.