Choose a large mixing bowl because the batter will double in bulk as it rises. Put the water in the mixing bowl and sprinkle in the yeast. Let stand to dissolve for 5 minutes. Whisk in the milk, butter, salt, sugar, and flours and beat until smooth. Cover the bowl with plastic wrap and let stand overnight in the refrigerator.
Preheat the waffle iron on its highest setting. Just before cooking the waffles, beat the eggs and baking soda into the batter. (The batter will be thin.) Pour ½ cup batter into the waffle iron. Bake until waffles are crisp and golden brown. Extra batter will keep for several days in the refrigerator.