@MagicBlueFrog I have just finalized the checking and it is an engine bug when dealing with reflections with split screen. I have just created a new project (4.17, 4.18 and 4.19) with 3rd person template, added Water Planes asset into it and left the TranslucentWater_Example map as default, added a 2nd PlayerStart and assigned both players with auto for player 0 and 1 respectivelly, created a game mode class including in the OnBegin event a CreatePlayer, set override for game mode with the new one created, and set split screen vertical.
When executed, while moving the player 0, the reflections on the water get messed on player 1. In translucent water, there is no Planar Reflections, there is no plugin, therefore no InfiniteOcean system, just the plain free Epic’s Water Planes.
On Monday morning I will submit a bug report, since is detected in the latest 3 engine versions, it might be even older. Once I get the number I will post it here, so others can vote for it to receive more attention from developers.