Is it possible to make water use a different material through a water mask or something similar?
Yes, but you’ll need to do it manually. First you need a Raster Overlay with the mask. If you have a GeoTIFF, for example, you can upload it to Cesium ion to tile as imagery, and then access it with a Cesium ion Raster Overlay. Next, you need a custom material that uses the mask value to select whether to render water or not. There’s no way to switch between two entirely separate materials (because both the water and non-water parts will be rendered in a single draw command), so you’ll need one material that chooses between the effects.
It does, but we haven’t added support for it to Cesium for Unity, yet.