Is your lake positioned within the WaterZone actor? You can click on it and extend the “Box Extents” X & Y.
Keep in mind that the ocean-island-lake-river actors are simply landscape brush tools. If they are outside of the WaterZone volume then you won’t see any water material. Yes, you’ll see a water plane outside of that volume but I think that plane is the lower resolution “far plane” that extends into the horizon.
How big is your landscape, and do you have another screenshot?