The reason this is occuring is due to your refraction. I suggest taking a look at the ‘Water Planes’ example content we have in the ‘Learn’ tab of the Epic Launcher. It provides three separate examples of water surface materials and how to account for things like refraction and opacity.
I have also written a tutorial for a water shader which takes you step by step so you can to set up the material on your own. If you end up getting lost or stuck the material is provided in a zipped folder at the end of the tutorial. You just need to connect the node inputs.
Water Shader Tutorial
Let me know if you have further questions or need additional assistance.