I see this [1] article re: identifying & tracking individuals watching illegal streams, but I fail to see how it could work technically. Wouldn't there need to be something installed &running locally to identify & report the watermark?[1]:
https://www.google.com/amp/www.foxnews.com/tech/2017/08/28/mystery-code-appeared-in-pirated-mayweather-vs-mcgregor-streams-report-says.amp.html