Preheat oven to 375° F. In large skillet, cook ground beef over high heat until no longer pink, about 10 minutes, stirring frequently to break up. Pour off any fat. Stir in marinara sauce and water. Set aside.
Squeeze spinach dry. In medium bowl, combine spinach, ricotta cheese, Parmesan cheese and egg; stir well. Spread 1 cup meat sauce over bottom of 13 x 9 x 2 inch baking dish. Top with 4 lasagna noodles, overlapping to fit. Top with half of ricotta mixture, spreading to cover noodles. Top with 1/3 of the Muenster cheese slices. Spoon on 1 cup meat sauce, spreading to cover. Repeat layering of noodles, ricotta, cheese slices and meat sauce. Top with remaining 4 noodles. Spoon on remaining meat sauce, spreading to cover noodles. Cover dish loosely with foil. Bake 30 minutes. Uncover and top with remaining cheese slices. Bake 20 minutes, or until hot and bubbly. Let stand 15 minutes before serving.