Combine oats, almond milk, water, and salt in a small saucepan over high heat until starting to boil, 1 to 2 minutes. Stir peanut butter into oatmeal until dissolved, 1 to 2 minutes. Continue cooking and stirring for 2 minutes more. Lower heat and stir in cinnamon; cook for 1 to 2 minutes.
Remove oatmeal from heat and stir in banana and vanilla extract.