Meanwhile, to a blender, add the cubed watermelon, pinch cayenne pepper, salt and sugar. Pulse until the watermelon is smooth. Add the watermelon puree through a strainer and discard the pulp. Pour the tomato water to the watermelon juice and mix. Divide the tomato/watermelon mixture between popsicle molds. Transfer to the freezer for about 1 hour. At the 1-hour mark, push the popsicle sticks into each mold. Freeze until solid, about 6 hours, ideally overnight. To remove, run the molds under warm water and then remove the popsicles.