It seems there is no way for Truesky to be visible from underwater using the water system in UE4 4.26. If you could access the truesky cubemap, you could render it to a sky dome when underwater, but that still isn’t possible. Trying to use the “Render to Cubemap” blueprint node will crash the engine.
If you are using Unreals native water I would assume (as I have not dove into it personally) that you would need to adjust the water material they are using to add in our trueSKY transparency nodes found within the plugin content.
For some background information on why this is its due to trueSKY being on its own custom render pass, and as such needing to be accounted for when using transparent materials (such as what I am assuming the Unreal’s water is).
We are aware of the render to Cubemap crash and have been looking into a fix for this.
Apologies if this is an inconvenience, or any of the information is incorrect as I am assuming a fair amount of information about the unreal water system here.