It finally dawned on me what your "water mark" actually is.
Click
HERE for pic.
Click
HERE for information.
Your "watermark" isn't water at all, its a
moire pattern (a visiual interference pattern) being generated between the two plastic layers of the digitizer. Between these two layers is a gel, when any kind of pressure is put upon the screen over a large area, this gel is displaced a bit and the two layers are forced closer together, allowing the creation of the moire pattern. On my Axim it looks a lot like an irregular "island" of a Mandelbrot Set, usually longer top to bottom, approximately in the center of the screen.
This happens to me occasionally when the foam lining of my PocketSolutions CoverUp is squeezed against my screen. It will normally clear itself up (you can actually see the pattern getting smaller).
If it is a bit more persistent, you can gently "massage" the screen with your thumbs from the outer edge towards the center, to redistribute the gel more quickly. Click the links above for more information.