Watermarks can be added in various ways. If you already have an existing watermark in form of a PDF, you could just use the overlay functionality (see Merging PDFs). Alternatively, a watermark can be added via CSS. The following sample CSS uses a page margin box to add the semi-transparent text "CONFIDENTIAL" above each page:
@page { @left-middle { content: "Confidential"; z-index: 100; font-family: sans-serif; font-size: 80pt; font-weight: bold; color: gray(0, 0.3); text-align: center; text-transform: uppercase; transform: rotate(-54.7deg); position: absolute; left: 0; top: 0; width: 100%; height: 100%; } }