A little known unknown secret in the industry is that there are much simpler watermarking systems than what people think.
Yes each copy is tagged with multiple watermarks visible and cryptomarks in both the visual and audio channels.
However the most common mark is the "cut" mark, the studio will have multiple copies with slightly different cuts of a couple of scenes including sometimes very slight post production changes (e.g. items that appear or do not appear in a given scene).
These marks are as simple as adding or removing a few frames from several scenes that do not change the context of the scene or the movie.
With 10-20 scenes that can easily be manipulated you can produce a large number of marked copies that cannot be wiped without knowing what scenes have been touched.
The studio usually produces a few versions which are scene marked and for the individual copy marking relies on several layers of cryptographic and traditional water markings.
This means that even if all of the technical watermarkings fail the studio still knowns roughly who leaked the video then pinpointing the source is much easier in most cases.
P.S.
Studios make multiple versions of movies with different cut tales also for other reasons such as to gauge audience responses to specific scenes or in cases where movies would have slight line differences for different regions.