Craft

Unmissable, and still readable underneath

The two requirements pull against each other, and most watermarks fail one of them badly.

Too faint

A mark nobody notices does not do its job. If a reader has to look for it, it is decoration.

Too heavy

A mark that fights the text makes the document annoying to read, and annoyed readers ask for a clean copy — which is how an unmarked version ends up in circulation after all.

Too wordy

One or two words. A sentence across the page is unreadable at any weight.

Diagonal across the middle is the convention for a reason: it crosses every column, survives cropping, and sits where nobody puts body text.