I'm not an expert so someone could correct me on this, but in my experience turbidity is often an issue, in that light penetration is insufficient in the types of water where this is most often needed.
The light doesnt have to reach all the water... it has to reach the molybdenum sulfide/copper catalyst in the first stage of the process so stirring should enable that
e.g., https://www.sciencedirect.com/science/article/pii/S004896971...
Still cool though.