Peel potatoes; slice lengthwise into 1/4-inch-thick slices.
Arrange half of sliced sweet potatoes in a lightly greased 13- x 9- x 2-inch baking dish. Sprinkle 1/2 cup sugar over sweet potatoes; sprinkle with cinnamon. Arrange remaining potatoes and lemon slices over top; sprinkle with remaining sugar and cinnamon. Pour water over sweet potatoes. Dot with butter.
Cover and bake at 350° for 30 minutes. Baste potatoes with syrup formed in bottom of baking dish. Bake, uncovered, an additional 10 minutes.