Every state has it's own chosen watermarks. Some use chemical watermarks.
Alaska uses Premier's ECO-ASSURE. That paper has a watermark manufactured into the sheet. This watermark, which is non-reproducible and cannot be digitally scanned or photocopied, gives election officials assurance that the printed ballot is the original source document issued.
https://aws.state.ak.us/OnlinePublicNotices/Notices/Attachment.aspx?id=93893