Cut watermelon into small chunks, discarding the rind. Place chunks in a blender and puree until smooth. In a large saucepan, place 1 C. of the watermelon puree, the sugar, lime zest, lime juice and apricot nectar (or orange juice). Cook mixture over medium-low heat, stirring often, until simmering. Pour in the remaining watermelon puree and stir well. Whisk in the salt and corn syrup until incorporated. Remove from heat and allow to cool for 10 minutes.Pour the mixture into a 9-inch metal cake pan, cover with plastic wrap and freeze for 4 hours or until firm. Remove from freezer, cut into 2- inch chunks and place in a food processor. Process until smooth, then transfer to a freezer-safe container. Let sit at room temperature for 5-10 minutes before serving.