Combine 1 cup water and sugar in a microwave-safe bowl and heat on high for 2 minutes; cool completely. In a blender, add sugar syrup, lime juice, mint and cilantro leaves, salt, crushed red pepper, lime rind, cucumber and blend until smooth. Pour mixture into an 11 x 7-inch glass baking dish; cover and freeze for 4 hours or until firm, stirring with a fork every 50 minutes. Remove mixture from freezer and scrape mixture with a fork until fluffy. Place watermelon cubes in a large bowl. Sprinkle with salt and toss to combine. Let stand for 10 minutes. Spoon 1 1/2 cups watermelon mixture into 6 bowls and top each serving with 1/2 cup of the granita. Serve.