Bring water to a boil in a saucepan. Stir in oats; cook until tender, 10 to 20 minutes. Stir heavy cream, brown sugar, butter, and cinnamon into the oats; simmer for 5 minutes. Add diced banana and walnuts; cook for 1 to 2 minutes more.
Remove oats from heat and stir in vanilla extract. Serve in bowls topped with sliced banana.