NovaOpacity is the setting that matters most
A watermark that's too dark makes the underlying page hard to read; one that's too light is easy to crop or edit out. A light-to-medium opacity is the right starting point for most uses — visible enough to clearly mark the document, faint enough that the actual content stays the primary thing a reader sees.
Push opacity higher only when the goal is discouraging reuse of the content itself, like a preview copy of a paid document — at that point readability of the watermark matters more than readability of the page underneath.
Text vs. image watermarks
A text watermark ("DRAFT," "CONFIDENTIAL," a company name) is the simplest and covers most everyday needs — marking a document as not-yet-final, or labeling internal-only paperwork. An image watermark is the better fit for branding, stamping an actual logo onto pages before sharing a document externally, or for a more distinctive mark than plain text can give.
Position and rotation
A diagonal watermark through the center of the page is the classic look for marking a document as a draft or sample — it's hard to crop out without losing page content, and it doesn't collide with header or footer text. A flat, corner-positioned watermark reads more like a stamp or logo placement and suits branding better than draft-marking.
If the page already has a header, footer, or page number in the corners, a center or diagonal placement avoids overlapping content you'd rather keep clearly readable.