Preheat oven to 350 degrees.
In a small bowl, combine cinnamon and 3 1/ 2 tablespoons granulated sugar.
Dice 3 plums and cut 2 into thin slices.
Toss diced plums in cinnamon sugar. Set aside. Toss slices in cinnamon sugar and set aside as well (separate from diced).
Whisk flour, baking powder and salt in a large bowl.
Combine remaining granulated sugar, ½ cup of brown sugar, melted butter, water, vanilla extract and eggs together in a medium-sized bowl. Stir the wet ingredients into the dry ingredients until well combined.
Pour 1/3 of the batter into a greased loaf pan. Spread half of the diced plums over the batter. Pour another 1/3 batter over the plums and top with the remaining diced plums. Pour the rest of the batter in and top with sliced plums.
Pour any remaining liquid from the plums over the top.
Bake for 1-1 ½ hours or until a toothpick comes out clean.