Welcome to the community! A little bit more information would help me answer your question. What type of data are you trying to upload? What is your use case - are you trying to upload terrain to cover large parts of the ocean or smaller bodies of water such as rivers or lakes?
In the meantime, here is a guide covering CesiumJS support for visualizing terrain and water effects.
I suspect that this guide will have a lot of the information that you are looking for. I also found this community forum post that discusses water masks in Cesium for Unreal.
-Sam