Drain pineapple, reserving juice. Set pineapple aside. Add water to juice (if needed) to measure 1 cup; pour into a large skillet. Add remaining ⅓ cup water, vinegar, soy sauce, brown sugar, and cornstarch; whisk until smooth.
Cook over medium heat until thick, stirring constantly. Add the meatballs and cook for 7-8 minutes. Add the green pepper and stir; cook an additional 12 minutes. Stir in the pineapple chunks and cook for 1-2 minutes or until pineapple is heated through.