Our US Government dollar bills are certainly watermarked, visibly, and I would guess that most every person in this country has at least 1 bill of some sort in their possession, so it could certainly be done.
Regarding the potential of all votes in every state being watermarked, I'd say that potential is zero. My guess is some watermarking is done in some states or precincts. My hope is that it was done where it was needed - in the contested states, and especially in their larger cities.
I'm not saying that it can't be done. Of course it can. But states control paper ballot specs right down to watermarking (again, CA has legal statutes covering watermarking).
The logistics of, say DHS, "secretly" mandating watermarks or anything else to state-owned and financed ballots would be problematic. That wouldn't stay secret for long.
And again, few if any states actually print or even mail ballots. The manufacture, printing, and mailing of ballots to voters is done entirely by just a handful of USA contractors.