I generate CHMs, but I think the same logic for watermarks applies to WebHelp.
1) You can create a Master Page with a watermark on it and apply the page to all/some topics in your Web Help (when you build or pre-build through the Topic List pod). I do this with a Draft watermark and it works great.
2) You can use the Format...Topic Borders and Shading option to apply a watermark to pages, one at a time.
The only caveat is that you have to create the watermark image yourself...though there are tons of "How To" tutorials on the Web to guide you.
Usually the way to add watermarks is by editing the CSS to include the watermark.
Assuming the CSS is associated with all topics, the watermark should then appear on all topics using the CSS file.
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