The question here is not to what product management advice to the Gemini team. The discussion here is whether the watermarking is noticeable.

The easy thing to do here would be to have 1000 questions, randomly assigning one half to an LLM with a watermark, and the other half without. Then show people pairs and say, "Which one seems watermarked?" (Or, "Which text seems more natural" or "Which is a better answer" or something like that.) If they come out equal, the watermark really is indiscernible, at least to most people.