For someone who isn’t familiar with Unreal this is tricky, but if you follow Kevin’s solution here: Custom water mask - #4 by Kevin_Ring
or pijan9’s solution here: Change Water color in cesium water material - #2 by pjiang9
Both of these tackle the issue you are and should solve your problem.
If you just wanted a height blend on the land, you could get away without using custom materials here at all. Unreal sample includes a MI_CesiumHeightBlend and you just have to set the materials on the Tileset in your scene directly. However you will lose the watermask. You could adjust the low altitude color on the material to blue and maybe achieve an affect you’re happy with.
For example, on Level 9, select the CesiumWorldTerrain tileset, then if you scroll through the details section you should find some materials under Rendering
You’ll notice that there is a water material here, that only works ( i believe) if the “material” set above it expects a water material.
I have already set mine to CesiumHeightBlend and that looks like this:
If I duplicate the material and set the low altitude colour to blue you get:
Obviously this is not as optimal as getting a water mask working on a custom material, but if aren’t able to get Kevin and Pijangs solution working for you, this may be an okay alternative.
Hope that helps!