Put cut watermelon into freezer bags and store in freezer for at least 5 hours, preferably overnight. This can be done weeks in advance.
Put the frozen watermelon into a blender. Add all the remaining ingredients and pulse until the mixture is thoroughly blended and creamy, with flecks of cilantro throughout. Pour into 4 ice cold pilsner glasses or other tall glasses. Garnish each glass with a cilantro sprig and a lime wheel. Serve.