This can be assembled ahead of time and refrigerated until ready to bake, but allow a little more baking time to be sure it has heated through.
In a large bowl, mix ricotta cheese, 1 cup mozzarella cheese, egg, spinach, salt, oregano, and pepper. In a greased 13x9-inch pan, layer 1 cup sauce, 3 of the noodles, and half of the cheese mixture. Repeat. Top with remaining noodles and sauce. Sprinkle with remaining 1 cup mozzarella. Pour water around the edges of the pan. Cover tightly with foil and bake at 350 degrees F for 1 hour, 15 minutes. Let stand for 15 minutes before serving.