Place the cranberry juice and sugar in a medium pot set over medium-high heat. Stir just until the sugar has dissolved. Remove from the heat and stir in the watermelon and lime juices.
Pour the mixture into your popsicle mould, insert the sticks and freeze until solid. Keep frozen until ready to serve.
Note: To prepare the watermelon juice, place cubes of peeled, seedless watermelon into a food processor or blender and pulse until smooth. Strain and firmly press through a fine strainer. You'll need about 6 cups of cubed watermelon to get 3 cups of juice.