Position rack in the center of the oven; preheat to 400°F. Butter a 2-quart, 2-inch deep baking dish or 10-inch glass or ceramic pie plate.
To make the filling: Combine cranberries, maple syrup, and water in medium saucepan. Bring to a simmer; cook for 3 minutes. Transfer 1/2 cup of berries, with some juice, to small bowl; set aside. Pour remaining berries and juice into baking dish; heat in oven for about 5 minutes.
To make the topping: Whisk flour, cornmeal, baking powder, and salt in medium bowl. Whisk egg, milk, butter, and vanilla in another medium bowl; add flour mixture and whisk until blended. Remove baking dish from oven and scrape batter over berries (no need to cover completely). Spoon reserved berries and juice randomly over batter. Bake 15-20 minutes until topping is golden brown, feels crusty, and springs back when you press it. Cool pudding for 5 minutes before serving (with desired topping).