Bring water to a boil in a large pot. Add sweet potatoes and cook until tender, 20 to 30 minutes; drain. Place sweet potatoes in a bowl and mash with a potato masher until smooth.
Preheat oven to 350 degrees F (175 degrees C). Grease a 9x5-inch loaf pan.
Stir sugar and oil together in a bowl until well mixed; beat in banana. Combine flour, baking soda, cinnamon, nutmeg, salt, and baking powder in a bowl. Alternate stirring the flour mixture and 1/3 cup water into the sugar mixture until combined. Mix in sweet potatoes and pecans; stir until batter is smooth. Pour batter into the prepared loaf pan.
Bake in the preheated oven until top is golden brown, about 1 hour.