I have a 22.5" WSM and I foil the water pan. I fill mine just about 1/2 way and it lasts the entire cook.
After closing the vents so the coals go cold I scoop out the ashes and transfer them to the water pan. The water soaks up most (to all) of the water and creates a kind of mud-like substance. Then I fold in the foil (bringing the seam ends together first) before folding it into a nice "pillow" that slips neatly into an empty briquette bag for the trash.