As far as I know, watermarks never took off because the technical complexity and lack of standardization. Nielsen researched and developed ACR technology but it never required watermarks.

When Nielsen first started collecting data (1950s), they handed out diaries to a group of around 5 000 selected households. Nielsen would then collect these diaries and extrapolate viewing patterns across the entire population.

After that came phone surveys. Next step (1980s) was the "people meter", a small box attached to the TV that could automatically detect which channel was tuned and which household members were in the room.

In the 1990s the set-top box made all of that redundant as the cable and satellite providers could directly see which channel every single box was tuned to at all times.