Preheat oven to 400°F. Grease a 9x13-inch baking dish with nonstick spray.
Whisk together the marinara sauce, water, Italian seasoning, salt, and pepper in a large measuring cup or bowl.
Pour a thin layer of the sauce mixture into the bottom of the prepared dish.
Arrange half of the frozen tortellini in a single layer over the sauce.
Pour about half of the remaining sauce mixture on top.
Sprinkle with ½ cup of mozzarella.
Arrange the remaining tortellini in a second layer. Pour the remaining sauce over top and press down gently so all the tortellini is coated. Sprinkle with the remaining mozzarella and finish with Parmesan.
Cover tightly with foil and bake for 15 minutes. Remove the foil and bake for another 10 to 15 minutes, until the tortellini is al dente. For browned, bubbly cheese, transfer to the broiler for 1 to 2 minutes.
Let stand 5 to 10 minutes before serving. Garnish with extra Parmesan and fresh herbs if desired.