To make the granita, bring water and sugar to a boil, remove from the heat and allow to cool. Mix with the juice and pour into a loaf pan. Place in freezer and stir with a fork every hour or so until mixture resembles frozen slush. To make the panna cotta, scald cream and milk together with sugar. Soften the gelatin in cold water, drain well, and add to hot milk. Strain and cool. To assemble: In a tall parfait glass, layer the watermelon chunks, the cream, more chunks and finally the granita.