In a microwave-safe bowl, melt marshmallows with water in the microwave. Add as much powdered sugar as can be stirred in (approx. 1 cup). Dust a smooth surface with powdered sugar. Knead in additional powdered sugar until only slightly sticky. Wrap in plastic wrap and let set overnight, room temperature. Work a small amount with shortening-greased hands. If still sticky, add more powdered sugar.