Hi there!
Just seeing if anyone else has run into the same issue (or if I’ve missed something!)
Cesium Water Masks work on the Cesium World Terrian, however not on the new Google 3D Tiles (despite there being Water Mask options for both).
‘Enable Water Mask’ is selected for both Terrian sets, however only the ‘flat’ CesiumWorldTerrian tiles display the water mask.