In a 3-qt. casserole dish, heat water and butter 1 ½ to 2 minutes on high until boiling. Stir in molasses. Heat 13 - 14 minutes on high until mixture reads 300 F when tested with a candy thermometer. Do not use candy thermometer in dish while the microwave oven is on. Quickly stir in peanuts and baking soda, stirring until soda foams. Pour onto well greased cookie sheet; spread thin. Let stand until candy is hardened. Break into small pieces.