Place ham slice in a 12- x 8- x 2-inch baking dish. Bake, uncovered, at 350° for 30 minutes.
Combine water, vinegar, sugar, and mustard; pour over ham. Bake, uncovered, an additional 30 minutes. Remove ham to a warm serving dish, reserving ham drippings.
Combine ham drippings and flour in a saucepan. Cook over medium heat, stirring constantly, until thickened. Serve sauce over warm ham.