Place a 9x9-inch metal baking pan in the freezer.  In a small saucepan over medium-high heat, bring the sugar and water to a boil.  Boil for 5 minutes.  Place in a large bowl and cool completely.  When cooled, add the watermelon puree, lemon juice, and rosemary.  Pour into the frozen baking pan, cover with foil, and return to the freezer.  Freeze, stirring occasionally, for 3 hours, or until partially frozen.  Place the mixture in a food processor with the metal blade attachment.  Process until smooth but still frozen.   Return the mixture to the baking pan, cover, and freeze for 3 hours longer, or until frozen.  Remove from the freezer 15 minutes before serving.  Scoop into dessert bowls.