Arrange the sliced apples into a 9x13-inch baking dish. Add the sugars, flour and cinnamon over the apple slices. Pour in the water and mix everything through until apples are evenly coated.
In a bowl, combine the oats, flour, sugars, cinnamon, salt, baking powder and melted butter. Mix well using a metal spoon or your fingertips, until the butter has soaked through the dry ingredients (about one minute of mixing or less). Sprinkle evenly over the apple filling.
Bake in preheated oven for 40 - 45 minutes, or until the crumb is golden and crisp and the apples are cooked to your liking.
Notes
Tip: Serve your delicious Apple Crumble with a cold scoop of vanilla ice cream or some warm custard.