What a watermark is for
A watermark does not prevent copying, and treating it as protection leads to disappointment. What it does is establish provenance and context: it marks a document as a draft so nobody acts on superseded figures, identifies the recipient so a leaked copy can be traced, or labels a file confidential so a reader knows the handling expectation before reading a word.
Those are genuinely useful, and largely achieved by the watermark being visible rather than by it being difficult to remove. A DRAFT stamp across every page prevents the most common document accident — someone printing a working version and treating it as final — at essentially zero cost.
Per-recipient watermarking, where each copy carries the name of the person it was sent to, is the one case where a watermark does deter behaviour. It does not stop a leak, but it makes the leaker identifiable, which changes the calculation for anyone considering it.
Legibility on both sides
The tension in every watermark is being noticeable enough to serve its purpose without making the document hard to read. Opacity between roughly 10 and 25 percent is the usual working range: below that it vanishes when printed on a laser printer, above that it competes with the text.
A diagonal orientation at around 45 degrees is conventional because it crosses text lines rather than running along them, so it stays legible as a whole while interfering with any individual line for only a short span. It also makes the mark harder to crop away, since it touches most of the page.
Consider the printed result as well as the screen. Light grey at 15 percent on screen may disappear entirely on a monochrome printer with toner saving enabled, and a colour watermark can print as an ambiguous mid-grey. If the document is likely to be printed, test one page before stamping two hundred.
Removal is straightforward, so plan around it
A watermark applied as a separate layer over page content can generally be removed, because it remains a distinct object in the file. Various tools will strip it, and even flattening does not help much — with a light watermark over plain text, image processing can often subtract it cleanly.
That is not an argument against watermarking, only against relying on it. If a document genuinely must not circulate, the control is access rather than annotation: encrypt it with a user password using the protect tool, or do not distribute it in a format the recipient keeps.
The useful mental model is that a watermark communicates intent to a cooperative reader and creates accountability for an uncooperative one. Both are worthwhile. Neither is a lock.