Fill a tea kettle with water and bring to a boil on the stove. Place a small piece of fabric over the spout of the tea kettle, to prevent steam from gushing out. When the water starts boiling, remove from the heat and point the steam directly at the water stain until it saturates the fabric and then let dry.
Wet the wedding dress slightly with water from a spray bottle, enough to dampen the fabric, but not completely saturate it. Then run your fingernail against the fabric, scraping at the stain or use a dull knife to scrape at the stain.
Dip a washcloth in water and dab it at the water stain, moving from the inside of the stain to the outside edges. This causes the water spots to move out and feathers the edges. Once you wet down the area, dry it slowly with a hair dryer on its lowest setting.
Dunk the stained portions of the polyester dress completely in water mixed with Woolite or another product designed for cleaning delicate clothing. Let the dress sit in the water for 5-10 minutes and then check the stain, as this should remove all traces. Be careful doing this if your dress has beading or elaborate detailing.
Press tissue paper down on top of the water stain, switching pieces whenever one becomes saturated. This method is typically used for removing water stains on carpeting, but can also be used on clothing. Let the tissue paper absorb all traces of the water and when dry, the stain should be gone.