Haha, nice, glad you managed to get rid of that message 
About the water disappearing when Player 1 moves away, you could try the following: select the water mesh, go to the Details panel, find the Rendering section, and increase the Bounds Scale value. Try 3 or 5 first to see if that fixes the issue. You can also enable “Never Distance Cull,” which tells Unreal that this component should not stop rendering based on its distance from the camera.
It could also be related to LODs (Level of Detail). LODs are simplified versions of a mesh that the engine uses when the object is farther away, to improve performance. The issue is that sometimes a distant LOD can have a different material, fewer polygons, or even UV problems, which can make the water look wrong or disappear at certain distances. You could check the LOD settings on the water’s Static Mesh.
Another possibility is that the issue isn’t with the water mesh itself, but with the Landscape LOD, as mentioned in that post. When the terrain changes LOD, its shape or depth can shift slightly, and that can sometimes create visual problems for surfaces that are very close to it, like water.