Heat oven to 375°F and lightly grease a 9x13-inch baking dish.
Boil spaghetti in well-salted water for 6 minutes, just shy of al dente; drain.
Warm olive oil in a large skillet over medium heat. Add garlic and cook 30 seconds.
Stir in marinara, water, Italian seasoning, and red pepper flakes. Simmer 2–3 minutes.
Fold in meatballs and cook 5 minutes, stirring once, until heated through.
Combine drained spaghetti with the sauce and half of each cheese. Season with salt and pepper.
Spread mixture in the baking dish and sprinkle with remaining mozzarella and Parmesan.
Cover with foil and bake 20 minutes. Uncover and bake 8–10 minutes until browned and bubbly.
Rest 5 minutes, then garnish with parsley and serve hot.