No news or roadmap commitment at the moment, but it did occur to me while re-reading this just now that there might be a way to do what you need even with current capabilities.
Basically, the idea is to create a GeoTIFF of your water mask, covering whatever area you need with whatever resolution is appropriate. Then upload it to Cesium ion to tile as imagery. Then, in Cesium for Unity, add a Cesium ion Raster Overlay and point it at the asset created from your GeoTIFF. At this point you should see the water mask drawn over the terrain.
The next step is to create a custom material that reads from the overlay texture and, instead of just blending it with the base color below, it instead renders a water effect for the pixels that the overlay indicates should be water.