A discussion on Yog-Sothoth on making watermarks jogged my memory of a technique I stumbled upon years ago.
1. Head to local big box store. Purchase one can of spray waterproofing in the footwear section and one light gauge plastic "For Sale" sign.
2. Laser print or photocopy document.
3. Cut a stencil of your intended watermark out of the "For Sale" sign. Place it on your paper and lightly apply the spray waterproofing, coating only the area with the watermark. Allow to dry.
3. Mist paper with the steam from a teapot. If you get too close to the mouth of the jet the heat may damage your inks, so hold your paper near the end of the visible plume. You just want to dampen the paper with the steam so the grain rises.
The steam will raise the grain of the paper except in the area that was waterproofed with the stencil. Tada! Instant watermark. Feel free to reverse the stenciling procedure if you want a raised watermark as opposed to an impressed one.
Silicone based products produce the best results, but any waterproofing spray (or even matte fixative) will work. You just need something that will keep the paper from absorbing any of the water vapor and swelling. The "watermark" will be notably shiny, but the application of some fine powder like charcoal dust or baking cocoa will tone it down.