Heat your oven to 425°F and coat a 13x9-inch baking dish with nonstick spray.
In the baking dish or a large bowl, combine the uncooked penne, marinara sauce, water, salt, pepper, and diced raw chicken, stirring everything evenly.
Cover the dish tightly with aluminum foil to trap moisture and bake for 30 minutes, allowing the pasta to soak up the liquid and the chicken to cook.
Remove the foil and stir the pasta gently; it will still be firm and somewhat liquidy, which is expected.
Scatter the shredded mozzarella evenly over the top, then sprinkle with the Panko breadcrumbs and grated Parmesan cheese.
Return the dish to the oven uncovered and bake for another 10 to 15 minutes until the cheese melts, the topping turns golden, the pasta is tender, and the chicken is fully cooked.
If desired, finish by sprinkling fresh herbs like basil or parsley over the casserole before serving.