Preheat oven to 400°F. Grease a 9 x 13-inch baking dish or lightly coat with nonstick cooking spray.
In a large measuring cup or bowl, whisk together the marinara sauce, water, and a few pinches of salt and pepper. Pour a small amount of the sauce mixture into the bottom of the prepared dish (just enough to coat in a thin layer).
Arrange about half of the frozen tortellini in a single layer in the dish. Pour about half of the remaining sauce mixture over top, and sprinkle with ½ cup of grated mozzarella.
Arrange the remaining tortellini in another layer in the pan. Pour the remaining sauce over top. Press down gently to make sure that all of the tortellini are coated in sauce. Sprinkle remaining mozzarella over top and sprinkle with Parmesan cheese.
Cover tightly and bake for 20 minutes. Remove cover and continue baking until the tortellini is al dente, about for 15-20 more minutes. Transfer the dish to the broiler for a couple of minutes if you like the cheese browned and crispy on top.
Let stand 5-10 minutes before serving. The pasta will continue to absorb some of the sauce. Garnish with additional Parmesan cheese and chopped fresh herbs, if desired.