In a small pot, cook water and sugar till the sugar is completely dissolved. Keep this simple syrup aside.
Place all ingredients in a blender and process till smooth.
This is an optional step but I like to strain the puree to catch any seeds or watermelon bits.
Pour into a metal baking pan and freeze mixture for 1 hour or until semi firm.
Using a fork, scrape granita to form icy flakes. Keep in freezer again.
Repeat this step for 2-3 times until you get nice ice crystals.
Serve in pretty glasses top with mint or a wedge of lime.