Hey @Rxmex,
Welcome to the forum! Nice work getting the water mask up and running! The boundary issue you’re describing is pretty common when mixing a custom material with Cesium’s water mask system.
I think what’s happening is that the altitude material and the water material are operating as two separate systems, which creates those unassigned gaps at coastlines and messy borders between the two. The cleaner fix is to integrate both into a single material using Cesium’s material layering system.
The tileset actor has three material slots: Material, Translucent Material, and Water Material. You should be able to point all three to the same custom material instance for seamless rendering.
@pjiang9 walked through this workflow recently in Change Water color in cesium water material. Related threads: Custom water mask and Rendering the water using ue4.
Let me know if this helped! The aviation GPS use case sounds interesting.
Cheers!