Not a true water mark unless you also have Acrobat pro.
You can add some password protection, under the security part of the pdf export window. Take a look at the options in Permissions on that window.
One thing you can do with a doc on only a few pages is to make some sort of watermark. Then add a new layer on the layers palette, put the watermark there, and leep the layer visible. Then make the pdf and send the pdf with that watermark. If everything is okay, you can turn off the watermark layer and send the pieces.
With longer pieces, You can put the watermark on a Master and apply to all pages and make the proof pdf.
If you're losing money, then Acrobat pro might be worth it as watermarking is a brezze in that.
Using OSX 10.3.9 on a G4