Be sure the piece of silk you use is white.  Never use colored silk to remove a stain.  Rubbing your silk with colored silk could cause the colors to transfer during the process of stain removal.  Try using a white pillow case. If you cannot find any white silk, you can try a piece of muslin or white cotton. Lie your stained garment on a flat surface.  Gently rub the piece of white silk over the stain to lift the water stain off your garment. Just like colors can transfer, so can the stain.  The goal is to transfer the stain from your silk garment to the cleaning patch. Pay attention to the grain pattern on your garment.  Always rub with or across the grain.  Rubbing at a diagonal will wear out your garment and could cause it to lose its shape.

Summary:
Find a piece of white silk. Rub the water mark with the white piece of silk. Avoid rubbing at a diagonal to the silk grain.