Heat oven to 375°F. Lightly oil a 9x13-inch baking dish.
Bring a large pot of salted water to a boil.
Parboil manicotti 6–7 minutes until flexible. Drain and cool on a greased sheet.
Stir ricotta, chicken, 1 cup mozzarella, 1/4 cup Parmesan, and beaten egg in a bowl.
Add garlic, Italian seasoning, salt, pepper, spinach, and red pepper. Mix until creamy.
Simmer marinara, water, and olive oil 3–4 minutes to warm and loosen.
Spread 1 cup sauce in the baking dish.
Pipe or spoon filling into shells. Arrange stuffed shells in the dish.
Spoon remaining sauce over shells. Top with remaining mozzarella and Parmesan.
Cover with foil. Bake 25 minutes.
Uncover and bake 10–15 minutes until bubbling and lightly browned. Rest 10 minutes.
Garnish with parsley and serve hot.