In a mixing bowl, combine the sugar and butter. Beat until fluffy. Add the egg and vanilla; stir until combined. Mix in the bananas and water. After stirred, add in flours, oats, baking powder, salt, and baking soda. Mix just until combined, finishing by hand-stirring.
Pour into a greased loaf pan. Bake in preheated oven for about 1 hour. Remove from the pan and cool completely before slicing and serving.