Unfortunately, I think you would have to modify the plugin itself to remove the dependency! You can find the plugin source code on Github here.
For starters, the CesiumForUnreal.uplugin file mentions the Water plugin, so you’ll want to remove it from there. You’ll also have to modify MI_CesiumThreeOverlaysAndClippingAndWater to remove the water effect from the material. I imagine there are other places you have to change too, but I haven’t looked into them closely.