Preheat the oven to 425°F.
Combine the meatballs, marinara sauce, water, uncooked pasta, salt, and crushed red pepper in a large baking dish and stir well so the pasta is evenly coated. Press the pasta down so it's submerged in the sauce.
Cover the dish securely with aluminum foil and bake for 30 minutes.
Remove the foil and give everything a good stir. Check that the pasta is al dente (cooked through but still with a slight bite); if not, re-cover and bake for another 5 to 10 minutes.
Take the dish out of the oven, remove the foil, and cover the casserole with a layer of mozzarella.
Bake uncovered for another 5 to 10 minutes, until the pasta is tender and the cheese is melted.
Let the casserole rest for five minutes before serving. Top with chopped fresh parsley or Parmesan, if desired.