Toast bread in a large shallow baking pan in middle of oven until just dry, 25 to 30 minutes. Cook onions, celery, and herbs in butter in a large heavy skillet over moderately low heat, stirring occasionally, until celery is softened, about 10 minutes. Stir together bread, vegetables, broth, water, and salt and pepper to taste, then cool completely, uncovered.
You can make stuffing 1 day ahead and keep it covered and chilled.