Watermarks track processing, not authorship
Anthropic is careful about this in its own documentation, and the care is going to get lost the moment a detector ships.
A detected mark means content may have been processed by Claude. It does not mean Claude wrote it.
The proofreading problem
Ask Claude to fix the grammar in an essay you wrote yourself and it regenerates the passage, marking it in the process.
What comes back is substantially your writing, carrying a mark indistinguishable from one on text Claude drafted from nothing. Reporting after the announcement found this was the single loudest complaint, and it came from writers and developers rather than from people trying to hide anything.
Who absorbs the cost
Non-native speakers polishing their own prose. Writers with accessibility needs. Students who write their own work and check it before submitting.
All of them produce marked text while doing nothing anyone would object to, and none of them can prove otherwise by pointing at the mark.
What actually protects you
Drafts. Version history, timestamped documents and commit logs demonstrate authorship in a way the absence of a watermark never could.
If a policy applies to you, disclosing assistance is far more defensible than concealing it. That is not a moral position so much as a practical one — disclosed help is a conversation, discovered concealment is an incident.