The documentation of watermarks is not correct. It states
A Watermark(t) declares that event time has reached time t in that stream, meaning that all events with a timestamps t’ < t have occurred.
whereas the JavaDocs which is aligned with implementation says
A Watermark tells operators that receive it that no elements with a
timestamp older or equal to the watermark timestamp should arrive at the
The documentation needs to be updated. Moreover, we need to carefully check that the watermark semantics are correctly described in other pages of the documentation and blog posts published on the Flink website.