one of our teams has an issue with the new Water Body River feature in their UE26.1 project in one particular level only. The river is visible in the regular Editor Viewport but when switching to Game Mode in the Editor most of the river simply disappears except a few parts in the Level.
For me, the solution was to move the WaterZone object back into the bounds of the waterbody again, which was a WaterBodyOcean in my case. Then it reappeared in Game Mode.
This method worked for me, moved the WaterBrushManager and WaterMeshActor much closer to where all my WaterBodyLakes where located, seems to have fixed the issue