Despite the fact that XtraReports doesn't support watermarks for ASP.NET applications, there are ways to avoid this restriction. One of these approaches is illustrated by this example.
To simulate text watermarks, you can create a DIV block and via css classes define its placement on a web page against the block containing the DocumentViewer. The key role here mainly belongs to the z-order property. Also, you can avoid the watermark's text selection by adding the OnSelectStart='return false;' into the watermark's DIV tag (however, this fails to work in Opera, while IE and Mozilla understand this clearly).
Be accurate when positioning the watermark's block, pay special attention to its dimensions. Also, a watermark's overall text appearance can be widely customized via css properties.